fix a grammar error

src/macro.c

@@ -162,7 +162,7 @@ expand(pic_state *pic, pic_value obj, struct syntactic_env *env)
     pic_error(pic, "unexpected value type");
     return pic_undef_value();	/* unreachable */
   }
-  /* logic falw (suppress warnings gcc will emit) */
+  /* logic flaw (suppress warnings gcc will emit) */
   abort();
 }