Remove newline at end of MWExceptionRenderer::getShowBacktraceError

Also add a period at the end of the sentence.

This changes the HTML comment from

<!-- Set $wgShowExceptionDetails = true; at the bottom
of LocalSettings.php to show detailed debugging
information
 -->

to

<!-- Set $wgShowExceptionDetails = true; at the bottom
of LocalSettings.php to show detailed debugging
information. -->

This is a follow-up to 842b7a1769 (T162315).

Change-Id: I45ff4f856ad1c0dc72066fce7771077ff4663785