Clean up E0368 and E0369 explanations

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0368.md

@@ -1,5 +1,7 @@
-This error indicates that a binary assignment operator like `+=` or `^=` was
-applied to a type that doesn't support it. For example:
+A binary assignment operator like `+=` or `^=` was applied to a type that
+doesn't support it.
+
+Erroneous code example:
 
 ```compile_fail,E0368
 let mut x = 12f32; // error: binary operation `<<` cannot be applied to

src/librustc_error_codes/error_codes/E0369.md

@@ -1,4 +1,5 @@
 A binary operation was attempted on a type which doesn't support it.
+
 Erroneous code example:
 
 ```compile_fail,E0369