docs: use pwsh title on Windows install snippet

Avoids the layout reflow caused by the 'In Windows PowerShell:' prose
line on the Windows tab. The title attribute labels the snippet inline
with the code fence.

Author: bartlomieju

runtime/index.md

@@ -50,9 +50,7 @@ curl -fsSL https://deno.land/install.sh | sh
 </deno-tab>
 <deno-tab value="windows" label="Windows">
 
-In Windows PowerShell:
-
-```powershell
+```powershell title="pwsh"
 irm https://deno.land/install.ps1 | iex
 ```