Fix typo in the draft release action

Author: jtpio

.github/actions/draft-release/action.yml

@@ -49,7 +49,7 @@ runs:
         export RH_POST_VERSION_SPEC=${{ inputs.post_version_spec }}
         export RH_DRY_RUN=${{ inputs.dry_run }}
         export RH_SINCE=${{ inputs.since }}
-        export RH_SINCE_LAST_STABLE=$${{ inputs.since_last_stable }}
+        export RH_SINCE_LAST_STABLE=${{ inputs.since_last_stable }}
 
         # Install Jupyter Releaser from git
         pip install -q git+https://github.com/jupyter-server/jupyter_releaser.git@v1