Fix a typo in Walker::display_element inline comment

[david.binda]

This is a very minor issue, but since I came across it while looking into some unrelated stuff, I thought it might be worth logging.

There should be no "s" at the end of children, since it's already the plural form of child (one of the irregular plurals).

Inline comment in question: https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-walker.php?rev=48590#L146

[sabernhardt]

Patch applies, and I didn't find any other instances of "childrens" (in src).

[SergeyBiryukov]

In 49509:

Docs: Fix typo in a comment in Walker::display_element().

Props david.binda, sabernhardt.
Fixes #51713.