fix: change "an matching id" to "a matching id" (vuejs#521)

Author: davidwarrington

src/guide/a11y-semantics.md

@@ -60,7 +60,7 @@ Though you might have seen labels wrapping the input fields like this:
 </label>
 ```
 
-Explicitly setting the labels with an matching id is better supported by assistive technology.
+Explicitly setting the labels with a matching id is better supported by assistive technology.
 :::
 
 #### aria-label