chore(slots): fix typo (vuejs#1557)

Fix a confusion between header and footer

Author: guillaumeduliscouet

src/guide/components/slots.md

@@ -405,7 +405,7 @@ Named scoped slots work similarly - slot props are accessible as the value of th
   </template>
 
   <template #footer="footerProps">
-    {{ headerProps }}
+    {{ footerProps }}
   </template>
 </MyComponent>
 ```