Fix typo in class-wp-customize-custom-css-setting.php

[Presskopp]

https://core.trac.wordpress.org/browser/trunk/src/wp-includes/customize/class-wp-customize-custom-css-setting.php?marks=206#L206

I think this one:
_Imbalanced/Unclosed character errors can be caused <code>content: "";</code> declarations. You may need to remove this or add it a custom CSS file._
should be:
Imbalanced/Unclosed character errors can be caused *by* <code>content: "";</code> declarations. You may need to remove this or add it *to* a custom CSS file.

[Presskopp]

fixing 2 typos

[Presskopp]

Sorry I left the underscores in the description

[westonruter]

In 39009:

Customize: Fix message grammar for custom_css validity error when imbalanced characters possibly due to content CSS property.

Props Presskopp.
See #35395.
Fixes #38564.