Remove reference to SSL3 in PORTING.md.

We don't support SSL3 at all now. Actually we haven't supported renego
SSL3 in even longer, so this was false even before yesterday.

Change-Id: Ie759477fa84099dd486c4c4604080ecf8ecdf434
Reviewed-on: https://boringssl-review.googlesource.com/29484
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

PORTING.md

@@ -123,7 +123,7 @@ feature, so BoringSSL rejects peer renegotiations by default.
 
 To enable renegotiation, call `SSL_set_renegotiate_mode` and set it to
 `ssl_renegotiate_once` or `ssl_renegotiate_freely`. Renegotiation is only
-supported as a client in SSL3/TLS and the HelloRequest must be received at a
+supported as a client in TLS and the HelloRequest must be received at a
 quiet point in the application protocol. This is sufficient to support the
 common use of requesting a new client certificate between an HTTP request and
 response in (unpipelined) HTTP/1.1.