design: 47781-parameterized-go-ast.md: remove back-ticks from title
design: 47781-parameterized-go-ast.md: remove back-ticks from title

I just noticed this causes the package paths to be elided from the page
title in my tab bar.

Change-Id: I9706cf8e704ae8f4f3c2f3fe32f5b739ded5a55d
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/343229
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/proposal,golang/proposal