clean up E0214 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0214.md

@@ -1,12 +1,17 @@
 A generic type was described using parentheses rather than angle brackets.
-For example:
+
+Erroneous code example:
 
 ```compile_fail,E0214
-fn main() {
-    let v: Vec(&str) = vec!["foo"];
-}
+let v: Vec(&str) = vec!["foo"];
 ```
 
 This is not currently supported: `v` should be defined as `Vec<&str>`.
 Parentheses are currently only used with generic types when defining parameters
 for `Fn`-family traits.
+
+The previous code example fixed:
+
+```
+let v: Vec<&str> = vec!["foo"];
+```