change script to assume dotnet is on the path

scripts/register-completions.ps1

@@ -1,7 +1,7 @@
 # PowerShell parameter completion shim for the dotnet CLI 
 Register-ArgumentCompleter -Native -CommandName dotnet -ScriptBlock {
      param($commandName, $wordToComplete, $cursorPosition)
-         C:\dev\github\cli\artifacts\win10-x64\stage2\dotnet.exe complete --position $cursorPosition "$wordToComplete" | ForEach-Object {
+         dotnet complete --position $cursorPosition "$wordToComplete" | ForEach-Object {
             [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
          }
  }