Clean up E0063 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0063.md

@@ -1,5 +1,6 @@
-This error indicates that during an attempt to build a struct or struct-like
-enum variant, one of the fields was not provided. Erroneous code example:
+A struct's or struct-like enum variant's field was not provided.
+
+Erroneous code example:
 
 ```compile_fail,E0063
 struct Foo {