SECURITY.md: update git-lfs default branch name

We update our link to the Git LFS project's main security documentation
in the git-lfs/git-lfs repository to use that repository's current
default branch name of "main".

Author: chrisd8088

SECURITY.md

@@ -1,4 +1,4 @@
 Please see
-[SECURITY.md](https://github.com/git-lfs/git-lfs/blob/master/SECURITY.md)
+[SECURITY.md](https://github.com/git-lfs/git-lfs/blob/main/SECURITY.md)
 in the main Git LFS repository for information on how to report security
 vulnerabilities in this package.