update .gitlab-ci.yml escape character while parsing a quoted scalar

.gitlab-ci.yml

@@ -12,7 +12,7 @@ build_docs:
 
   image: python:3.8.10-slim
   rules:
-    - if: "$CI_COMMIT_BRANCH =~ /^(master|devel)$/ || $CI_COMMIT_TAG =~ /^v\d+\.\d+\.\d+$/"
+    - if: "$CI_COMMIT_BRANCH =~ /^(master|devel)$/ || $CI_COMMIT_TAG =~ /^v\d+.\d+.\d+$/"
       when: always
       variables:                              # Override IMAGE_LABEL defined
           IMAGE_LABEL: "${CI_COMMIT_BRANCH}_${CI_COMMIT_SHORT_SHA}"  # at the job level.