[flang][runtime] Fix error message

A literal % character in an error message needs to be doubled
so that it appears in the output of snprintf().

Differential Revision: https://reviews.llvm.org/D132158

flang/runtime/namelist.cpp

@@ -329,7 +329,7 @@ static bool HandleComponent(IoStatementState &io, Descriptor &desc,
     }
   } else {
     handler.SignalError("NAMELIST component reference of input group item %s "
-                        "has no name after '%'",
+                        "has no name after '%%'",
         name);
   }
   return false;