design: fix typo in generics implementation document
design: fix typo in generics implementation document

Correct the markdown syntax for the second link in the "Generics Implementation - GC Shape Stenciling" document.

This addresses a typo recently introduced in CL 387214.

Change-Id: I96f743b8ebd32ad0ee37118182ddce6161eafbc5
GitHub-Last-Rev: 1a666c4e4385c6c305ef53fed824a88eb240fd53
GitHub-Pull-Request: golang/proposal#43
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/390454
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal