version-control/git/README.md: Update link to SWC contributing page

Catch up with the swcarpentry/website -> swcarpentry/site transition,
specifically 86ba191 (Replaced contributing.html page with contrib/
directory containing separate pages for different ways to contribute,
2013-08-22).

diff --git a/version-control b/version-control
index afae66408a7e27889ee502113b9faee17259ecab..58a493425a892c2696b2aa203ec3b5994417d61b 160000 (submodule)
--- a/version-control
+++ b/version-control
@@ -1 +1 @@
-Subproject commit afae66408a7e27889ee502113b9faee17259ecab
+Subproject commit 58a493425a892c2696b2aa203ec3b5994417d61b