SECURITY.md: use the new Git for Windows snapshots URL (#5402)

The `SECURITY.md` document mentions Git for Windows' snapshots and
helpfully provides a link.

For almost eight years, Git for Windows' snapshots were hosted [on Azure
Blobs](https://wingit.blob.core.windows.net/files/index.html).

As of a combination of PRs
(git-for-windows/git-for-windows-automation#109,
git-for-windows/gfw-helper-github-app#117),
snapshots are not only now built and deployed via GitHub Actions instead
of Azure Pipelines (and ARM64 artifacts are now included, too), they are
also hosted [on
GitHub](https://github.com/git-for-windows/git-snapshots/releases/),
with the main page being hosted [on GitHub
Pages](https://github.com/git-for-windows/git-snapshots/commits/gh-pages).

Therefore, the original link now redirects to a new location (which is
also a lot easier to remember): http://gitforwindows.org/git-snapshots.
Let's adjust the link to link there directly.

This is a companion PR of
git-for-windows/git-for-windows-automation#111
and of git-for-windows/build-extra#589.

Author: dscho

SECURITY.md

@@ -56,7 +56,7 @@ Note: there is currently a bug in the "Check daily for updates" code, where it m
 
 ## Snapshot versions ('nightly builds')
 
-Git for Windows also provides snapshots (these are not releases) of the current development as per git-for-Windows/git's `master` branch at the [Snapshots](https://wingit.blob.core.windows.net/files/index.html) page. This link is also listed in the footer of the [Git for Windows](https://gitforwindows.org/) home page.
+Git for Windows also provides snapshots (these are not releases) of the current development as per git-for-Windows/git's `master` branch at the [Snapshots](https://gitforwindows.org/git-snapshots/) page. This link is also listed in the footer of the [Git for Windows](https://gitforwindows.org/) home page.
 
 Note: even if those builds are not exactly "nightly", they are sometimes referred to as "nightly builds" to keep with other projects' nomenclature.