Bug 1561166 [wpt PR 17440] - Fix markdown to use powershell syntax highlighter, a=testonly

Automatic update from web-platform-tests
Fix markdown to use powershell syntax highlighter
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wpt-commits: f7ee963089151bd746c15acb72e2a65fb92ebacc
wpt-pr: 17440

testing/web-platform/tests/README.md

@@ -46,7 +46,7 @@ For example, on most UNIX-like systems, you can setup the hosts file with:
 
 And on Windows (this must be run in a PowerShell session with Administrator privileges):
 
-```bash
+```powershell
 python wpt make-hosts-file | Out-File $env:systemroot\System32\drivers\etc\hosts -Encoding ascii -Append
 ```