gitlab-ci.yml: fix update-docs job

The rules engine doesn't handle quotes in the normal sense.
Quotes are only allowed on literals, not variable references.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

.ostc-ci/gitlab-ci.yml

@@ -76,7 +76,7 @@ update-docs:
   trigger: OSTC/OHOS/docs
   rules:
     # Update the documentation when things land in the default branch.
-    - if: '$CI_BRANCH_NAME == "$CI_DEFAULT_BRANCH"'
+    - if: '$CI_BRANCH_NAME == $CI_DEFAULT_BRANCH'
       changes:
         # React to changes to the docs directory.
         - docs/**/*