docs: fix end of tag style (#1579)

Based on good practices.

docs/guide/custom-blocks.md

@@ -76,7 +76,7 @@ This is the documentation for component B.
 <!-- ComponentA.vue -->
 <template>
   <div>
-    <ComponentB/>
+    <ComponentB />
     <p>{{ docs }}</p>
   </div>
 </template>