Docs: Standardize capitalization of Ajax throughout core documentation per the core spelling guide.

Ajax, while considered an acronym for Asynchronous JavaScript and XML, is most commonly capitalized only in the first character.

Part props ocean90.
See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@38028 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-background.php

@@ -410,7 +410,7 @@ if ( current_theme_supports( 'custom-background', 'default-color' ) )
 	}
 
 	/**
-	 * AJAX handler for adding custom background context to an attachment.
+	 * Ajax handler for adding custom background context to an attachment.
 	 *
 	 * Triggered when the user adds a new background image from the
 	 * Media Manager.