Fix links in REFERENCE.md

The string `#{` was causing any yard links later in the file not to be
rendered (e.g. the literal "{Haml::Helpers}" is left in the html).

Close the open brace so yard creates the links correctly.

REFERENCE.md

@@ -1098,7 +1098,7 @@ and might compile to
 
     <p>This is scrumptious cake!</p>
 
-Backslashes can be used to escape `#{` strings,
+Backslashes can be used to escape `#{}` strings,
 but they don't act as escapes anywhere else in the string.
 For example: