Improve parameter name for wp_get_theme()

[johnbillion]

The name of the first parameter for wp_get_theme() is misleading. It's called $stylesheet when in fact you're passing in the name of the theme's template directory, not the stylesheet.

The function is a wrapper for the WP_Theme class which gets its parameter names correct.

I guess stylesheet is the common notation for theme's directory name throughout the theme-related functions.

That's how it's described in WP_Theme:
http://core.trac.wordpress.org/browser/tags/3.5/wp-includes/class-wp-theme.php#L86

It's also used as a parameter in get_theme_root(), get_theme_root_uri(), get_raw_theme_root(), switch_theme(), and wp_customize_url(), so 22972.patch​ would make that less consistent.