Add missing word in ProxyMode description

Change-Id: I62ac26fd680fd94a245c6b34af6edcb87abfebed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461037Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Chris Sharp <csharp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815190}

components/policy/resources/policy_templates.json

@@ -3884,7 +3884,7 @@
 
        If you choose to use:
          * <ph name="PROXY_MODE_ENUM_FIXED_SERVERS">fixed_servers</ph> = Fixed proxy servers. You can specify further options with <ph name="PROXY_SERVER_POLICY_NAME">ProxyServer</ph> and <ph name="PROXY_BYPASS_LIST_POLICY_NAME">ProxyBypassList</ph>. Only the HTTP proxy server with the highest priority is available for ARC-apps.
-         * <ph name="PROXY_MODE_ENUM_PAC_SCRIPT">pac_script</ph> =  A .pac proxy script. Use <ph name="PROXY_PAC_URL_POLICY_NAME">ProxyPacUrl</ph> to the URL to a proxy .pac file.
+         * <ph name="PROXY_MODE_ENUM_PAC_SCRIPT">pac_script</ph> =  A .pac proxy script. Use <ph name="PROXY_PAC_URL_POLICY_NAME">ProxyPacUrl</ph> to set the URL to a proxy .pac file.
 
       Leaving the policy unset lets users choose the proxy settings.