Docs: Improve the `prepare()` function inline docs

[dilipbheda]

I've reviewed the wpdb::prepare() function and made the following improvements to its inline documentation:

• Removed the duplicate mention of the $args parameter.
• Cleaned up the description by removing an unnecessary hyphen from the $query parameter.

[SergeyBiryukov]

Hi there, thanks for the patch!

The current description seems correct as is:

• The $args and ...$args parameters aim to differentiate between calling the method with an array of arguments to substitute into the query's placeholders and passing a ​variable number of individual arguments. The former part of the description should not be removed.
• The sprintf()-like wording is correct, the hyphen is used to ​join the two words together as an adjective.