Add single quotes around monospaced in less

Add single quotes around monospaced in less

Follow-Up: I3e3a7c110d4ceae8c1b65b35bf17b34fed7f68f8

Failed in

11:45:13 Running "stylelint:src" (stylelint) task
11:45:15 >>
/home/jenkins/workspace/mediawiki-core-npm-node-4.3/src/resources/src/mediawiki/mediawiki.debug.less
failed:
11:45:15 >> Line 98, column 15: Expected single quotes around font-family
name "monospaced" (font-family-name-quotes) (error)
11:45:15

Which this patch fixes.

Caused I278f6ef063d37c1b6325da76edc9f34b01c27087 to fail which this also
fixes.

Change-Id: I10f64ec5517176a43c292599e4bdb1ad7098929d