Specify TrueType font format as 'truetype' rather than 'ttf'

Summary:
I've been experimenting with delivering a TrueType-only KaTeX build for our mobile apps. I couldn't
get anything to render properly until I flipped the `format` specifier from `ttf` to `truetype`.
Though I can't find a definitive source on this, all the examples I've seen online use `truetype`
over `ttf`, and flipping from `ttf` to `truetype` fixed my own issues.

It's unlikely that this has been much of a problem in practice, since so many browsers now support
WOFF (I'm considering a TrueType-only build since it's the least common denominator across Android
4.3+ and mobile Safari).

Test Plan:
- To force TrueType rendering, comment out the `.use-eot`, `.use-woff2`, and `.use-woff` lines in `fonts.less`.
- Run `make serve`.
- Open up the demo page.
    - Verify that the demo text is rendered as before (only the parens are noticeably different in the demo?).

Reviewers: kevinb, emily

Reviewed By: emily

Subscribers: benkomalo

Differential Revision: https://phabricator.khanacademy.org/D29274

static/fonts.less

@@ -18,7 +18,7 @@
 }
 
 .use-ttf(@family, @family-suffix) when (@use-ttf = true) {
-    src+: url('@{font-folder}/KaTeX_@{family}-@{family-suffix}.ttf') format('ttf')
+    src+: url('@{font-folder}/KaTeX_@{family}-@{family-suffix}.ttf') format('truetype')
 }
 
 .generate-suffix(@weight, @style) when (@weight = normal) and (@style = normal) {