Clarify to install GCM for Windows for WSL

Previously it wasn't clear whether to install GCM for Linux or Windows when using WSL until later in the document.

Now the first step explicitly says GCM for Windows.

docs/wsl.md

@@ -41,7 +41,7 @@ If you wish to use GCM inside of WSL _without installing Git for Windows_
 you must complete additional configuration so that GCM can callback to Git
 inside of your WSL installation.
 
-Start by installing the [latest GCM ⬇️][latest-gcm]
+Start by installing the [latest GCM for Windows⬇️][latest-gcm]
 
 _Inside your WSL installation_, run the following command to set GCM as the Git
 credential helper: