Correct very minor grammatical error in section 8.3

The sentence in 8.3:

"CONNECT is primarily used with HTTP proxies to __established__ a TLS session with a server for the purposes of interacting with https resources."

... does not read. I believe it should be:

"CONNECT is primarily used with HTTP proxies to __establish__ a TLS session with a server for the purposes of interacting with https resources."

This pull request fixes this incredibly minor issue.