Fix function parameter parsing past 2 params. (#2542)

The comment on FunctionParameterFinish is actually correct (`1. FunctionParameter`), just a typo in the implementation. It specifically failed parsing at the comma after the 2nd param, regardless of whether there were more params.

toolchain/parser/parser.cpp

@@ -1090,7 +1090,7 @@ auto Parser::HandleFunctionParameterFinishState() -> void {
 
   if (ConsumeListToken(ParseNodeKind::ParameterListComma, TokenKind::CloseParen,
                        state.has_error) == ListTokenKind::Comma) {
-    PushState(ParserState::PatternAsFunctionParameter);
+    PushState(ParserState::FunctionParameter);
   }
 }