Clean up E0124 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0124.md

@@ -1,5 +1,6 @@
-You declared two fields of a struct with the same name. Erroneous code
-example:
+A struct with two fields having the same name has been declared.
+
+Erroneous code example:
 
 ```compile_fail,E0124
 struct Foo {