Add missing close quote to no-missing-keys.md. (#52)

Author: jlebar

docs/rules/no-missing-keys.md

@@ -32,7 +32,7 @@ localization codes:
 <template>
   <div class="app">
     <!-- ✗ BAD -->
-    <p>{{ $t('hi) }}</p>
+    <p>{{ $t('hi') }}</p>
     <!-- ✗ BAD -->
     <p v-t="'hi'"></p>
     <!-- ✗ BAD -->
@@ -91,4 +91,4 @@ const i18n = new VueI18n({
 
 /* ✓ GOOD */
 i18n.t('hello')
-```
+```