Administration: Add punctuation to "If you change this, we will send you an...

Administration: Add punctuation to "If you change this, we will send you an email" message on General Settings, Network Settings, and Profile screens.

Props kinjaldalwadi, donmhico.
Fixes #49235.
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git-svn-id: https://core.svn.wordpress.org/trunk@46943 1a063a9b-81f0-0310-95a4-ce76da25c4cd