Fix casing of Umb.PropertyEditorUi.Tiptap

This is case sensitive, at least in some contexts, so updating to it's correct casing.

Author: AndyButland

15/umbraco-cms/fundamentals/backoffice/property-editors/built-in-umbraco-property-editors/rich-text-editor/README.md

@@ -1,6 +1,6 @@
 # Rich Text Editor
 
-`Schema Alias: Umbraco.RichText` `UI Alias: Umb.PropertyEditorUi.TipTap`
+`Schema Alias: Umbraco.RichText` `UI Alias: Umb.PropertyEditorUi.Tiptap`
 
 `Returns: HTML`