doc: fixing link in keepalive docs (#2755)

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

fixes #2754 
diff --git a/Documentation/keepalive.md b/Documentation/keepalive.md
index 118ac66..c101680 100644
--- a/Documentation/keepalive.md
+++ b/Documentation/keepalive.md
@@ -35,7 +35,7 @@
 ### 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.