Fix @staticvar in the docblock of wp_rand()

[birgire]

The inline documentation for the wp_rand() has

* @staticvar bool $external_rand_source_available

but there's no such static variable in use. The changeset is [34981].

So it looks like a typo and should be:

* @staticvar bool $use_random_int_functionality

Good-first-bug?

Reference:

https://core.trac.wordpress.org/browser/tags/4.9.1/src/wp-includes/pluggable.php#L2315

[chetan200891]

Created patch to fix @staticvar

[birgire]

@chetan200891 thanks for the patch, it looks good to me

[SergeyBiryukov]

In 42377:

Docs: Correct @staticvar entry for wp_rand().

Props chetan200891, birgire.
Fixes #42796.