Replaced '_Object2World' with 'unity_ObjectToWorld'

Hi I just started learing unity shaders, - https://www.youtube.com/watch?v=epixwRw80MM

But when creating a difuse lightinig shader with single directional light

everytime _Object2World is replaced by unity_ObjectToWorld with a upgrade message on top

I tried searching for it but the bug is issue is closed in latest version of unity

My Unity ver - 5.4.0f3

 // Upgrade NOTE: replaced '_Object2World' with 'unity_ObjectToWorld'
 
 // Upgrade NOTE: replaced '_Object2World' with 'unity_ObjectToWorld'
 
 Shader "Unlit/Custom_Diffuse"
 {
     Properties {
         // Texture field in inspector where we assign texture to the model
         _MainTexture("Main Texture",2D) = "white"{}
     }
 
     SubShader {        
         PASS {
             CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
 
                 fixed4 _Color;
                 sampler2D _MainTexture;
                 float4 _LightColor0;
                 struct appData {
                     float4 vertex : POSITION;
                     float3 normal : NORMAL;
                     float2 texCoord : TEXCOORD0;
                 };
 
                 struct v2f {
                     float4 pos : SV_POSITION;
                     float3 normal : NORMAL;
                     float2 texCoord : TEXCOORD0;
                 };
 
                 v2f vert (appData IN) {
                     v2f OUT;
                     OUT.pos = mul(UNITY_MATRIX_MVP,IN.vertex);
 
                     // Directly used from model space to modelspace
                     // OUT.normal = IN.NORMAL;
                     // float4 (IN.normal,0.0) - normal is a float3 type variable declared in appData
                     // But we need float4 so this line converts float3 to float4 type casting we can say
                     //mul() to convert the normal from model space to world space as light is in world space
                     // _Object2World is a unity internal variable
                     //mul().xyz - swizling the resultant of mul is float4, this converts to float3
                     OUT.normal = mul(float4(IN.normal,0.0),unity_ObjectToWorld).xyz;
                     // just pass the texture coodinates of the UV map to the semantics in fragment
                     // Directly used from model space to modelspace
                     OUT.texCoord = IN.texCoord;
                     return OUT;
                 }
 
                 fixed4 frag(v2f IN) : COLOR{
                     // wraps the texture data to the UV coordinates recieved from the vertex function
                     float4 texColor = tex2D(_MainTexture,IN.texCoord);
 
                     float3 normalDirection = normalize(IN.normal);
                     float3 lightDirection = normalize(_WorldSpaceLightPos0.xyz);
 
                     float3 diffuse = _LightColor0.rgb * max(0.0,dot(normalDirection,lightDirection));
 
                     return _Color * texColor * float4(diffuse,1);
                 }
             ENDCG
         }
     }
 }