CURLOPT_SSLVERSION.3: fix typos and consistent spelling

Use TLS vX.Y throughout the document, instead of TLS X.Y, as that was
already done in all but a few cases. Also fix a few typos.

Closes #3076
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>
Reviewed-by: Daniel Stenberg <daniel@haxx.se>