Actually apply monospacing to the monospaced font preference example

Summary: See T551. We don't apply the default monospacing rules to the example,
so if you don't have a custom font selection you don't see the default
accurately.

Test Plan: Deleted my preference, saw an accurate default. Set my preference to
"14px impact", ensured it was respected in applications.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, nh

Differential Revision: 1035