multi: grammar fix in comment

After 'must', the verb is used without 'to'. Correct: "must" or "have
to"

Closes #8368

lib/multi.c

@@ -2168,8 +2168,8 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
           Curl_posttransfer(data);
           drc = multi_done(data, result, FALSE);
 
-          /* When set to retry the connection, we must to go back to
-           * the CONNECT state */
+          /* When set to retry the connection, we must go back to the CONNECT
+           * state */
           if(newurl) {
             if(!drc || (drc == CURLE_SEND_ERROR)) {
               follow = FOLLOW_RETRY;