Fix spelling mistakes in //docs.

docs/proxy_auto_config.md

 # Proxy Auto Config Using WPAD
 
 Most systems support manually configuring a proxy for web access, but this is
 cumbersome and kind of techical, so Chrome also supports
 [WPAD](http://en.wikipedia.org/wiki/Web_Proxy_Autodiscovery_Protocol) for proxy
 configuration (enabled if "automatically detect proxy settings" is enabled on
 Windows).
 
 ## Problem
 
 Currently, WPAD is pretty slow when we're starting up Chrome - we have to query
 the local network for WPAD servers using DNS (and maybe NetBIOS), and we wait
 all the way until the resolver timeout before we try sending any HTTP requests
 if there's no WPAD server. This is a really crappy user experience, since the
-browser's basically unuseable for a couple of seconds after startup if
+browser's basically unusable for a couple of seconds after startup if
 autoconfig is turned on and there's no WPAD server.
 
 ## Solution
 
 There's a couple of simplifying assumptions we make:
 
 *   If there is a WPAD server, it is on the same network as us, and hence likely
     to respond to lookups far more quickly than a random internet DNS server
     would.
 *   If we get a lookup success for WPAD, there's overwhelmingly likely to be a
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     reconfiguration.
 
 Here's what the **proposed** lookup policy looks like in practice:
 
 *   If there's no WPAD server on the network, we try to do a lookup for WPAD,
     time out after 100ms, and disable WPAD.
 *   If there's a WPAD server and our lookup for it answers in under 100ms or
     it's explicitly configured (via a custom PAC URL), we use that WPAD server.
 *   If there's a WPAD server and our lookup for it answers after 100ms, we time
     out and do not use it until a network change.