Add leading example sentence in C6284 warning reference

Author: Rageking8

docs/code-quality/c6284.md

@@ -19,6 +19,8 @@ Code analysis name: `OBJECT_AS_STRING_ARGUMENT_TO_FORMAT_FUNCTION`
 
 ## Example
 
+The following example generates C6284:
+
 ```cpp
 #include <atlbase.h>
 #include <string>