docs: update Homebrew install instructions

GCM has migrated from the microsoft/homebrew-git tap to the
Homebrew/homebrew-cask tap. Updating the install instructions accordingly.

Author: ldennington

docs/install.md

@@ -14,22 +14,21 @@ you have installed this using Homebrew, this installation will be unlinked
 #### Install
 
 ```shell
-brew tap microsoft/git
-brew install --cask git-credential-manager-core
+brew install --cask git-credential-manager
 ```
 
 After installing you can stay up-to-date with new releases by running:
 
 ```shell
-brew upgrade git-credential-manager-core
+brew upgrade --cask git-credential-manager
 ```
 
 #### Uninstall
 
 To uninstall, run the following:
 
 ```shell
-brew uninstall --cask git-credential-manager-core
+brew uninstall --cask git-credential-manager
 ```
 
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