fix(deps): bump ep_plugin_helpers range to ^0.6.1 (the published version) (#185)

Follow-up to #184.

Etherpad's plugin installer (live-plugin-manager-backed) resolves the
dep range's lower-bound version literally — `^0.6.0` makes it ask npm
for `ep_plugin_helpers@0.6.0`, which was never published (the auto-
publish workflow bumped straight from 0.5.3 to 0.6.1). CI on master
has been red since #184 merged for this reason. Same fix applied to
ep_font_color and ep_font_size (#143). Anchor to the actual published
version so the install resolves.

Author: JohnMcLear

package.json

@@ -34,7 +34,7 @@
     "url": "https://github.com/ether/ep_headings2/issues"
   },
   "dependencies": {
-    "ep_plugin_helpers": "^0.6.0"
+    "ep_plugin_helpers": "^0.6.1"
   },
   "devDependencies": {
     "eslint": "^8.57.1",