[gnome-shell-extensions] README: Fix typo on instructions to move to main branch

[Marco Trevisan <marcotrevi src gnome org>]

commit 8de89a44a43aba85df15a4bdd3bca04b3f5e3872
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Wed Jul 21 19:50:34 2021 +0200

    README: Fix typo on instructions to move to main branch

 README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/README.md b/README.md
index fb8be6b..4759ce6 100644
--- a/README.md
+++ b/README.md
@@ -74,7 +74,7 @@ GSettings key.
 The default development branch is `main`. If you still have a local
 checkout under the old name, use:
 ```sh
-git checkout matser
+git checkout master
 git branch -m master main
 git fetch
 git branch --unset-upstream