Error msg fix.

src/mx2cc/parser.monkey2

@@ -189,7 +189,7 @@ Class Parser
 				Case "field"
 					If flags & DECL_IFACEMEMBER Error( "Interfaces cannot have fields" )
 					If flags & DECL_EXTENSION Error( "Type extensions cannot have fields" )
-					If fileScope Error( "Fields must be declared within a class, struct or interface" )
+					If fileScope Error( "Fields may only be declared within a class or struct." )
 					ParseVars( decls,flags )
 				Case "local"
 					Error( "Locals can only be declared in a statement block" )