docs: use a human-readable README title

Replace the `ep_special_characters` placeholder heading with "Special
Character Picker for Etherpad" so newcomers can see at a glance what
the plugin does.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JohnMcLear

README.md

@@ -5,8 +5,7 @@
 > uses a browser-side `require()` that Etherpad's current esbuild bundler
 > doesn't expose. Tracking fix in [#87](https://github.com/ether/ep_special_characters/issues/87).
 
-# ep_special_characters
-
+# Special Character Picker for Etherpad
 TODO: Describe the plugin.
 
 ## Installation