tools: update to acorn@7.1.1

Update acorn to 7.1.1 in the dev dependencies for the markdown linter.

Refs: https://app.snyk.io/vuln/SNYK-JS-ACORN-559469
Signed-off-by: Rich Trott <rtrott@gmail.com>

PR-URL: #32259
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

Author: Trott