release process: Fix _ in the index files of release candidates

Manually tested with `source scripts/ci/build.sh; create_index_html rc2` where rc2 is the folder from 0.1.2rc2.

--
MOS_MIGRATED_REVID=108630774
diff --git a/scripts/ci/build.sh b/scripts/ci/build.sh
index be026ec..5da4dfd 100755
--- a/scripts/ci/build.sh
+++ b/scripts/ci/build.sh
@@ -276,7 +276,7 @@
   # Second line is to trick hoedown to behave as Github
   create_index_md "${@}" \
       | sed -E 's/^(Baseline.*)$/\1\
-/' | sed 's/^   + / - /' \
+/' | sed 's/^   + / - /' | sed 's/_/\\_/g' \
       | "${hoedown}"
 }