configmgr: dont dereference invalid iterator

i think the idea here was to print the key and not the iterator at or
past .end()

src/config/ConfigManager.cpp

@@ -1720,7 +1720,7 @@ static bool parseModeLine(const std::string& modeline, drmModeModeInfo& mode) {
         if (it != flagsmap.end())
             mode.flags |= it->second;
         else
-            Debug::log(ERR, "invalid flag {} in modeline", it->first);
+            Debug::log(ERR, "invalid flag {} in modeline", key);
     }
 
     snprintf(mode.name, sizeof(mode.name), "%dx%d@%d", mode.hdisplay, mode.vdisplay, mode.vrefresh / 1000);