[Chore] Fix a complation error due to new Universum

Problem: retryAfterInfo doesn't compile because the type of
readMaybe got more polymorphic in Universum.

Solution: specify explicitly that the argument is Text.
N.B. In the previous version it was String (due to the old type of
readMaybe), but Text is generally preferable, so we're changing it.

src/Xrefcheck/Verify.hs

@@ -768,7 +768,9 @@ checkExternalResource followed config@Config{..} link
           other -> throwError $ ExternalResourceSomeError $ show other
       where
         retryAfterInfo :: Response a -> Maybe RetryAfter
-        retryAfterInfo = readMaybe . decodeUtf8 <=< L.lookup hRetryAfter . responseHeaders
+        retryAfterInfo =
+          readMaybe @_ @Text . decodeUtf8 <=<
+          L.lookup hRetryAfter . responseHeaders
 
     checkFtp :: URI -> Bool -> ExceptT VerifyError IO ()
     checkFtp uri secure = do