Update ca1872.md (#43739)

Author: willibrandon

docs/fundamentals/code-analysis/quality-rules/ca1872.md

@@ -90,12 +90,12 @@ class HelloWorldEncoder
 
     public string Encode()
     {
-        return Convert.ToHexString(data);
+        return Convert.ToHexString(_data);
     }
 
     public string EncodeToLower()
     {
-        return Convert.ToHexStringLower(data);
+        return Convert.ToHexStringLower(_data);
     }
 }
 ```
@@ -108,11 +108,11 @@ Class HelloWorldEncoder
     Private ReadOnly _data As Byte() = Encoding.ASCII.GetBytes("Hello World")
 
     Public Function Encode() As String
-        Return Convert.ToHexString(data)
+        Return Convert.ToHexString(_data)
     End Function
 
     Public Function EncodeToLower() As String
-        Return Convert.ToHexStringLower(data)
+        Return Convert.ToHexStringLower(_data)
     End Function
 End Class
 ```