all.bash: link to prettier install page
all.bash: link to prettier install page

Change-Id: I73efcea136731bcb80a08cdc3702ba62e65b138f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/237977
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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