Networks and Sites: Update the link text for "Debugging a WordPress Network"...

Networks and Sites: Update the link text for "Debugging a WordPress Network" support article in `ms_not_installed()`.

Props Presskopp.
Fixes #50326.
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git-svn-id: https://core.svn.wordpress.org/trunk@47688 1a063a9b-81f0-0310-95a4-ce76da25c4cd