docs: Add pnpm workspace flag (-w) to Oxc setup docs (#12655)

### Description

[The oxc guide](https://turborepo.dev/docs/guides/tools/oxc) installs
dependencies at the workspace root, but by default pnpm warns in
workspaces unless `-w` is used.

This updates the commands to include `-w` so users can copy/paste
without hitting the warning.

Author: mattjoll

apps/docs/content/docs/guides/tools/oxc.mdx

@@ -40,7 +40,7 @@ First, install oxlint in your repository:
 <Tab value="pnpm">
 
 ```bash title="Terminal"
-pnpm add --save-dev oxlint
+pnpm add --save-dev -w oxlint
 ```
 
 </Tab>
@@ -133,7 +133,7 @@ Install oxfmt as a dev dependency:
 <Tab value="pnpm">
 
 ```bash title="Terminal"
-pnpm add --save-dev oxfmt
+pnpm add --save-dev -w oxfmt
 ```
 
 </Tab>