Fix minor wart with pre block closing (wikitext, 172a51c)

Under some circumstances (unnested pre block followed by two consecutive newlines) we would emit a block that looked like this:

<pre>foo </pre>

Rather than like this:

<pre>foo</pre>

While the former displayed fine in the browser, the latter looks better in the HTML source.

Signed-off-by: Greg Hurrell <greg@hurrell.net>