Fix shell syntax error

Author: huddlej

.github/workflows/run-usa-models.yaml

@@ -57,7 +57,7 @@ jobs:
             /repos/${HUB_OWNER}/${HUB_REPO}/pulls \
             -f title="Add blab models for ${DATE}" \
             -f body="" \
-            -f head="nextstrain:blab-${DATE}"
+            -f head="nextstrain:blab-${DATE}" \
             -f base=main \
             -F draft=true
         env: