Update Git for Windows screenshot link

Author: ridgunn

docs/install.md

@@ -241,7 +241,7 @@ dotnet tool uninstall -g git-credential-manager
 [gcm-credstores]: credstores.md
 [gcm-wsl]: wsl.md
 [git-for-windows]: https://gitforwindows.org/
-[git-for-windows-screenshot]: https://user-images.githubusercontent.com/5658207/140082529-1ac133c1-0922-4a24-af03-067e27b3988b.png
+[git-for-windows-gcm-screenshot]: ./img/git-for-windows-gcm-screenshot.png
 [latest-release]: https://github.com/git-ecosystem/git-credential-manager/releases/latest
 [linux-uninstall]: linux-fromsrc-uninstall.md
 [linux-validate-gpg-debian]: ./linux-validate-gpg.md#debian-package