Add a `gclient sync` to merge_request.md

When switching between release branches, one needs to `gclient sync` in
order to be able to build Chromium. This was not documented, and bit me
more than once :) This CL adds that line to remind future me (and
others) of that.

Change-Id: I839b2cc5211c88ba80c6551eac36fb5777dcaa01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4055508
Reviewed-by: Ian Clelland <[email protected]>
Commit-Queue: Yoav Weiss <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1075903}
diff --git a/docs/process/merge_request.md b/docs/process/merge_request.md
index b1c70ac..3f1e1411 100644
--- a/docs/process/merge_request.md
+++ b/docs/process/merge_request.md
@@ -194,6 +194,7 @@
 $ git checkout -b BRANCH_NAME refs/remotes/branch-heads/####
 $ git cl upstream branch-heads/####
 $ git cherry-pick -x COMMIT_HASH_MAIN
+$ gclient sync
 ```
 
 From here, your environment should be ready to adjust the change as required;