tools: fix a $ref typo in pull-cryptpad-upstream script

The $REF variable was incorrectly used as lowercase $ref
and so the git ref argument wasn't "seen" by the script.

tools/pull-cryptpad-upstream.sh

@@ -2,7 +2,7 @@
 REPO="https://github.com/xwiki-labs/cryptpad.git"
 PREFIX=cryptpad
 REF="${1}"
-if [ -z "${ref}" ]; then
+if [ -z "${REF}" ]; then
     printf "usage: $0 <ref>\n" 1>&2
     printf "ref: main, tag, branch or commit hash\n" 1>&2
     exit 1