Update GCM screenshot image for installation (#2154)

Added the new screenshot for the latest version under the img folder.
Updated the image file reference in `install.md`

Author: ridgunn

docs/img/git-for-windows-gcm-screenshot.png

@@ -151,7 +151,7 @@ manager.
 GCM is included with [Git for Windows][git-for-windows]. During installation
 you will be asked to select a credential helper, with GCM listed as the default.
 
-![image][git-for-windows-screenshot]
+![image][git-for-windows-gcm-screenshot]
 
 ---
 
@@ -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