Use : rather than . in example description

Co-authored-by: Guillaume Gomez <[email protected]>

Author: pickfire

src/librustc_error_codes/error_codes/E0749.md

@@ -18,7 +18,7 @@ Negative impls are not allowed to have any items. Negative impls declare that a
 trait is **not** implemented (and never will be) and hence there is no need to
 specify the values for trait methods or other items.
 
-One way to fix this is to remove the items in negative impls.
+One way to fix this is to remove the items in negative impls:
 
 ```
 # #![feature(negative_impls)]