Merge pull request #13892 from edx/jibsheet/fix-problem-builder-version

problem-builder's tags are vX.Y.Z not X.Y.Z

requirements/edx/edx-private.txt

@@ -8,7 +8,7 @@
 
 # For Harvard courses:
 -e git+https://github.com/gsehub/xblock-mentoring.git@4d1cce78dc232d5da6ffd73817b5c490e87a6eee#egg=xblock-mentoring
-git+https://github.com/open-craft/problem-builder.git@2.6.0#egg=xblock-problem-builder==2.6.0
+git+https://github.com/open-craft/problem-builder.git@v2.6.0#egg=xblock-problem-builder==2.6.0
 
 # Oppia XBlock
 -e git+https://github.com/oppia/xblock.git@9f6b95b7eb7dbabb96b77198a3202604f96adf65#egg=oppia-xblock