docs(selection): fix className documentation (#418)

Co-authored-by: Jan Thurau <[email protected]>

Author: arikchakma

.changeset/many-nails-argue.md

@@ -0,0 +1,5 @@
+---
+'tiptap-docs': patch
+---
+
+Fix Selection extension className documentation

src/content/editor/extensions/functionality/selection.mdx

@@ -50,13 +50,13 @@ new Editor({
 
 ### className
 
-The class that is applied to the focused element.
+The class that is applied to the current selection.
 
-Default: `'has-focus'`
+Default: `'selection'`
 
 ```js
 Selection.configure({
-  className: 'focus',
+  className: 'selection',
 })
 ```