chore(AIP-151): fix link to aip-163 (#1362)

Author: devrelm

aip/general/0151.md

@@ -59,7 +59,7 @@ rpc CreateBook(CreateBookRequest) returns (google.longrunning.Operation) {
 - APIs with messages that return `Operation` **must** implement the
   [`Operations`][lro] service. Individual APIs **must not** define their own
   interfaces for long-running operations to avoid non-uniformity.
-- If an RPC supports a [validate-only mode](aip-163), the response to a
+- If an RPC supports a [validate-only mode][aip-163], the response to a
   validation request **must** be one of the following:
   - A successful response with an `Operation` which is already complete, with
     the `done` field set to `true`, and a valid (but potentially empty) response