Use correct leading slash in common.ps1 repo root resolution

Author: benbp

eng/common/scripts/common.ps1

@@ -1,6 +1,6 @@
 # cSpell:ignore Apireview
 # cSpell:ignore Onboarded
-$RepoRoot = Resolve-Path "${PSScriptRoot}..\..\..\.."
+$RepoRoot = Resolve-Path "${PSScriptRoot}/../../../"
 $EngDir = Join-Path $RepoRoot "eng"
 $EngCommonDir = Join-Path $EngDir "common"
 $EngCommonScriptsDir = Join-Path $EngCommonDir "scripts"