Clean up E0463 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0463.md

@@ -1,4 +1,6 @@
-A plugin/crate was declared but cannot be found. Erroneous code example:
+A plugin/crate was declared but cannot be found.
+
+Erroneous code example:
 
 ```compile_fail,E0463
 #![feature(plugin)]