Fix typo in documentation.

Change-Id: I4f42113c0c44209347dede29e768844299e214b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613751Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660540}

net/docs/proxy.md

@@ -160,7 +160,7 @@ button on
 will not give feedback that the bad proxies were cleared, however capturing a
 new NetLog dump can confirm it was cleared.
 
-## Arguments are passed to `FindProxyForURL(url, host)` in PAC scripts
+## Arguments passed to `FindProxyForURL(url, host)` in PAC scripts
 
 PAC scripts in Chrome are expected to define a JavaScript function
 `FindProxyForURL`.