design/16339-alias-decls.md: fix formatting of syntax productions
design/16339-alias-decls.md: fix formatting of syntax productions

Use bold face to highlight syntax changes (rather than "red" for which
there is no support in markup).

For golang/go#16339.

Change-Id: I2556056d1d45d1c32f504643a564dc9c165b7f27
Reviewed-on: https://go-review.googlesource.com/25048
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal