AIP-193: fix the broken link to AIP-4221.

Author: estarter

aip/general/0193.md

@@ -71,7 +71,7 @@ message. The errors themselves **must** still be represented with a
 ## Further reading
 
 - For which error codes to retry, see [AIP-194](https://aip.dev/194).
-- For how to retry errors in client libraries, see [AIP-4221][].
+- For how to retry errors in client libraries, see [AIP-4221][https://aip.dev/client-libraries/4221].
 
 ## Changelog