Add leading example sentence in C3551 error reference

Author: Rageking8

docs/error-messages/compiler-errors-2/compiler-error-c3551.md

@@ -15,6 +15,8 @@ The leading return type in [trailing return type](../../cpp/functions-cpp.md#tra
 
 ## Example
 
+The following example generates C3551:
+
 ```cpp
 // C3551.cpp
 // compile with: /c