docs(AIP-194): fix broken link (#1508)

Author: dbolduc

aip/general/0194.md

@@ -30,7 +30,7 @@ would cause unintended state changes.
 **Note:** This AIP does not cover client streaming or bi-directional streaming.
 
 **Note:** For client side retry behavior in the client libraries: see
-[AIP-4221][].
+[AIP-4221](https://aip.dev/4221).
 
 ### Retryable codes