[Editorial] Fix grammatical typo (#712)

Author: antosart

index.bs

@@ -2413,8 +2413,8 @@ Content-Type: application/reports+json
       <a http-header>Content-Security-Policy</a>: <a>font-src</a> https://example.com/
     </pre>
 
-    Fetches for the following code will return a network errors, as the URL
-    provided do not match `font-src`'s <a>source list</a>:
+    Fetches for the following code will return a network error, as the URL
+    provided does not match `font-src`'s <a>source list</a>:
 
     <pre highlight="html">
       &lt;style&gt;