docs: update configure command for install from source

The current install from source instructions are out of date - they still
recommend using `git-credential-manager-core configure` rather than
`git-credential-manager configure`. Update the documentation to instruct
users to run the correct command.

Author: ldennington

docs/install.md

@@ -126,7 +126,7 @@ Download and run the script:
 
 ```shell
 curl -L https://aka.ms/gcm/linux-install-source.sh | sh
-git-credential-manager-core configure
+git-credential-manager configure
 ```
 
 **Note:** You will be prompted to enter your credentials so that the script