Use .res.mjs in installation instructions to align with create-rescript-app

Author: fhammerschmidt

pages/docs/manual/v11.0.0/installation.mdx

@@ -50,7 +50,7 @@ bun create rescript-app
   ```
 - If you selected the "basic" template, simply run it with:
   ```sh
-  node src/Demo.res.js
+  node src/Demo.res.mjs
   ```
 
 That compiles your ReScript into JavaScript, then uses Node.js to run said JavaScript.

pages/docs/manual/v12.0.0/installation.mdx

@@ -49,7 +49,7 @@ bun create rescript-app
   ```
 - If you selected the "basic" template, simply run it with:
   ```sh
-  node src/Demo.res.js
+  node src/Demo.res.mjs
   ```
 
 That compiles your ReScript into JavaScript, then uses Node.js to run said JavaScript.