change deprecated slot-scope to v-slot

Directive `slot-scope` is deprecated as of Vue v2.6.0. See [here](https://vuejs.org/v2/guide/components-slots.html) for details. Currently tiptap depends on Vue ^2.6.10. So this commit introduces new directive `v-slot` on README.md and examples.

examples/Components/Routes/History/index.vue

@@ -1,7 +1,7 @@
 <template>
   <div class="editor">
-    <editor-menu-bar :editor="editor">
-      <div class="menubar" slot-scope="{ commands, isActive }">
+    <editor-menu-bar :editor="editor" v-slot="{ commands, isActive }">
+      <div class="menubar">
 
         <button
           class="menubar__button"