Rename the newly added action 'after_theme_change' to 'after_switch_theme' so...

Rename the newly added action 'after_theme_change' to 'after_switch_theme' so it matches the 'switch_theme' action, props scribu, see #17979

git-svn-id: https://core.svn.wordpress.org/trunk@18656 1a063a9b-81f0-0310-95a4-ce76da25c4cd