edk2-TemplateSpecification: Use github.ref_name

Use github.ref_name instead of potential multiple matches
from branches that contain github.ref.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>

Author: mdkinney

.github/workflows/gitbook-action.yml

@@ -14,8 +14,7 @@ jobs:
       uses: actions/checkout@v2
     - name: Get Branch
       run: |
-        raw=$(git branch -r --contains ${{ github.ref }})
-        branch=${raw/  origin\/}
+        branch=${{ github.ref_name }}
         pubdir=${branch/master/draft}
         pubdir=${pubdir////-}
         echo "ON_PUSH_BRANCH_NAME=$branch" >> $GITHUB_ENV