Docs: replace reference to renamed $allowedxmlnamedentities global variable in wp_kses_xml_named_entities function's docblock

[david.binda]

The r48072 introduced a wrongly named $allowedxmlnamedentities global variable (
instead of correct $allowedxmlentitynames ), which was later fixed r52229, where almost all instances of $allowedxmlnamedentities were replaced by $allowedxmlentitynames, however the old name of the global variable is still present in the docblock for the wp_kses_xml_named_entities function. See https://core.trac.wordpress.org/browser/trunk/src/wp-includes/kses.php?rev=52326#L1928

[SergeyBiryukov]

In 52639:

Docs: Correct @global tag in wp_kses_xml_named_entities().

This updates the variable name in the DocBlock to the correct one.

Follow-up to [48072], [52229].

Props david.binda.
Fixes #54899.