Fix broken links (#916)

Author: blue-hope

aip/general/0134.md

@@ -165,7 +165,7 @@ rpc UpdateBook(UpdateBookRequest) returns (google.longrunning.Operation) {
   be if the RPC was not long-running).
 - Both the `response_type` and `metadata_type` fields **must** be specified.
 
-**Note:** Declarative-friendly resources (AIP-128) **should** use long-running
+**Note:** Declarative-friendly resources ([AIP-128][]) **should** use long-running
 update.
 
 ### Create or update
@@ -207,7 +207,7 @@ The user **must** have the update permissions to call `Update` even with
 `allow_missing` set to `true`. For customers that want to prevent users from
 creating resources using the update method, IAM conditions **should** be used.
 
-**Note:** Declarative-friendly resources (AIP-128) **must** expose the
+**Note:** Declarative-friendly resources ([AIP-128][]) **must** expose the
 `bool allow_missing` field.
 
 ### Etags
@@ -268,6 +268,7 @@ so.
 [aip-203]: ./0203.md
 [create]: ./0133.md
 [state fields]: ./0216.md
+[aip-128]: ./0128.md
 [required]: ./0203.md#required
 [optional]: ./0203.md#optional