Update generate-docs.ps1

Author: jfversluis

build/scripts/generate-docs.ps1

@@ -1,11 +1,11 @@
 param(
-    [string]$branch = "master",
+    [string]$branch = "main",
     [string]$token
 )
 
 # Yes, this is ignoring the parameter passed in by VSTS; we don't need the
 # parameter, but we can't remove it until all the active branches don't need it
-$branch = "master"
+$branch = "main"
 
 $mdoc = '..\..\build\tools\mdoc\mdoc.exe'
 $docsUri = "https://$token@github.com/xamarin/Xamarin.Forms-api-docs.git"