doc: ran prettier on frontend.md
doc: ran prettier on frontend.md

A stray newline is removed from running prettier via all.bash.

Change-Id: I777b816c1355a909f9275e12d8fcb22dc9d578f0
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/270239
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite