Correct the parameter type for the `$stylesheet` parameter in the `mce_css` filter documentation.

Fixes #34721
Props hnle


git-svn-id: https://develop.svn.wordpress.org/trunk@35660 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-editor.php

@@ -572,7 +572,7 @@ final class _WP_Editors {
 				 *
 				 * @since 2.1.0
 				 *
-				 * @param array $stylesheets Comma-delimited list of stylesheets.
+				 * @param string $stylesheets Comma-delimited list of stylesheets.
 				 */
 				$mce_css = trim( apply_filters( 'mce_css', implode( ',', $mce_css ) ), ' ,' );