Fix URL

I thought published advisories would automatically be promoted from

    https://github.com/owner/repo/security/advisories/GHSA-xxx

to

    https://github.com/advisories/GHSA-xxx

but it looks like this only happens for GitHub-reviewed advisories.
Getting reviewed apparently requires a CVE, and I'm not a huge fan of
the CVE system so I'm not going to press GitHub's "Request CVE" button
(although I don't mind if someone else does).

With this issue plus the known issue of the rustsec ID not being
assigned until publication, it looks like I'm going to have to publish
a 1.3.1 just to change the links.

src/quoting_warning.md

@@ -247,7 +247,7 @@ Numeric escapes would solve this as well.
 Versions of this crate before 1.3.0 did not quote `{`, `}`, and `\xa0`.
 
 See:
-- <https://github.com/advisories/GHSA-r7qv-8r2h-pg27>
+- <https://github.com/comex/rust-shlex/security/advisories/GHSA-r7qv-8r2h-pg27>
 - (TODO: Add Rustsec link)
 
 ## Solved: `!` and `^`