Clean up E0647 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0647.md

@@ -1,4 +1,5 @@
-It is not possible to define `start` with a where clause.
+The `start` function was defined with a where clause.
+
 Erroneous code example:
 
 ```compile_fail,E0647