PHPDoc correction for wp_upload_bits()

[leogermani]

In version 5.3 the error control operator was removed from the call to fwrite.

This means that the if the $bits paramater is anything but a string, this function will throw a Warning, because $bits is passed directly to fwrite and fwrite expects it to be a string.

The inline docs declare $bits as mixed, I think we should change it to string.

[SergeyBiryukov]

Thanks for the ticket! This was already corrected in [46660] as a part of #48303.