Text change/correction adding a missing period to wp_die message.

[Presskopp]

This is probably the smallest possible change you can make, adding a '.'

I compared all the instances of wp_die messages, like wp_die(__('Invalid plugin page')); in admin.php, L.225 and it turns out to be the only one without a period, so here the patch to add it. I'm very proud of it ;-)

Other examples:

'Link not found.'
'Invalid user ID.'

[Presskopp]

wow, https://core.trac.wordpress.org/changeset/2019 from 2004-12-30

[SergeyBiryukov]

In 39941:

Text Changes: Add a period to the only wp_die() message that didn't have it: "Invalid plugin page".

Props Presskopp.
Fixes #39664.