Administration: Change code tags to regular text for dimensions in “Site Icon” help text.

Follow-up to [57713], [58927].

Props juanfra, sabernhardt, niravsherasiya7707, manhar, mukesh27, vgnavada, wpdevsxyz, kevinrobles, SergeyBiryukov.
Fixes #64792.

git-svn-id: https://develop.svn.wordpress.org/trunk@62412 602fd350-edb4-49c9-b593-d223f7449a82