Adjust propose instructions to "checkout -b ticket/{id} origin/{branch}"

src/main/java/com/gitblit/wicket/pages/propose_git.md

@@ -1,6 +1,6 @@
     git clone ${url}
     cd ${repo}
-    git checkout -b ${reviewBranch} ${integrationBranch}
+    git checkout -b ${reviewBranch} origin/${integrationBranch}
     ...
     git push --set-upstream origin ${reviewBranch}