fix hookshot commandPrefix jinja2

roles/matrix-bridge-hookshot/templates/config.yml.j2

@@ -32,7 +32,7 @@ github:
     # (Optional) Default options for GitHub connections.
     #
     ignoreHooks: {{ matrix_hookshot_github_ignore_hooks }}
-    commandPrefix: {{ matrix_hookshot_github_command_prefix }}
+    commandPrefix: "{{ matrix_hookshot_github_command_prefix }}"
     showIssueRoomLink: {{ matrix_hookshot_github_show_issue_room_link }}
     prDiff: {{ matrix_hookshot_github_pr_diff }}
     includingLabels:{{ matrix_hookshot_github_including_labels }}