doc: fixing link in keepalive docs (#2755)

The link for enforcement policy in the keepalive docs is wrong.

fixes #2754

Documentation/keepalive.md

@@ -35,7 +35,7 @@ for details.
 ### Enforcement policy
 
 [Enforcement
-policy](https://godoc.org/google.golang.org/grpc/keepalive#ServerParameters) is
+policy](https://godoc.org/google.golang.org/grpc/keepalive#EnforcementPolicy) is
 a special setting on server side to protect server from malicious or misbehaving
 clients.