Recommend the use of https and mention git on building/checkout.html

Recommend the use of https and mention git on building/checkout.html
https://bugs.webkit.org/show_bug.cgi?id=93657

Reviewed by Mark Rowe.

This page was quite out-dated. Stop mentioning about how to install subversion on OS X 10.4
because the paragraph right above it refers to buliding/tools.html for that.

Recommend the use of HTTPS connection for the Subversion checkout and also mention how to
checkout using a Git client.

* building/checkout.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@125208 268f45cc-cd09-0410-ab3c-d52691b4dbfc