used BOOST_FALLTHROUGH; instead of a comment

Author: Roy Bellingan

include/boost/json/basic_parser_impl.hpp

@@ -686,7 +686,7 @@ parse_value(const char* p,
             // or just leave it as is
             if(BOOST_JSON_UNLIKELY(p == sentinel()))
                 return maybe_suspend(p, state::val2);
-            // intentional fallthrough
+            BOOST_FALLTHROUGH;
         case ' ':
         case '\t':
         case '\n':