remove quotes from character class

Things inside a character class should not be quoted, since the quotes would be picked up
as characters to match. Thankfully the compiler warns now about these mistakes.

src/Perl6/Grammar.nqp

@@ -3495,7 +3495,7 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
             | ':' <?before \s | '{'> <!{ $*QSIGIL }> <args=.arglist>
             ]
             || <!{ $*QSIGIL }> <?>
-            || <?{ $*QSIGIL }> <?['.']> <?>
+            || <?{ $*QSIGIL }> <?[.]> <?>
         ] <.unsp>?
     }