fix typescript docs

docs/src/docPages/guide/typescript.md

@@ -38,11 +38,13 @@ import { Command, Extension } from '@tiptap/core'
 
 declare module '@tiptap/core' {
   interface Commands {
+    customExtension: {
       /**
        * Comments will be added to the autocomplete.
        */
       yourCommand: (someProp: any) => Command,
     }
+  }
 }
 
 const CustomExtension = Extension.create({