docs: switch Windows install fence to shell for syntax highlighting

The powershell language tag has no highlighter in the docs site, leaving
the snippet uncolored. shell renders with the same shell-style colors as
the macOS/Linux fences.

Author: bartlomieju

runtime/index.md

@@ -57,7 +57,7 @@ curl -fsSL https://deno.land/install.sh | sh
 </deno-tab>
 <deno-tab value="windows" label="Windows">
 
-```powershell title="pwsh"
+```shell title="pwsh"
 irm https://deno.land/install.ps1 | iex
 ```