docs: Update Invoke-WebRequest to Start-BitsTransfer (#373)

Author: tim bangma

README.md

@@ -43,7 +43,7 @@ Set-ExecutionPolicy RemoteSigned
 Then, you can run:
 
 ```ps
-Invoke-WebRequest https://git.io/rustlings-win | Select-Object -ExpandProperty Content | Out-File $env:TMP/install_rustlings.ps1; Unblock-File $env:TMP/install_rustlings.ps1; Invoke-Expression $env:TMP/install_rustlings.ps1
+Start-BitsTransfer -Source https://git.io/rustlings-win -Destination $env:TMP/install_rustlings.ps1; Unblock-File $env:TMP/install_rustlings.ps1; Invoke-Expression $env:TMP/install_rustlings.ps1
 ```
 
 To install Rustlings. Same as on MacOS/Linux, you will have access to the `rustlings` command after it.