Remove unnecessary double dashes in npm commands

Author: MarcusCaspeco

src/content/docs/en/guides/environment-variables.mdx

@@ -93,13 +93,13 @@ This allows you to run the dev server or build your site connecting to different
  <Fragment slot="npm">
     ```shell
     # Run the dev server connected to a "staging" API
-    npm run astro dev -- --mode staging
+    npm run astro dev --mode staging
 
     # Build a site that connects to a "production" API with additional debug information
-    npm run astro build -- --devOutput
+    npm run astro build --devOutput
 
     # Build a site that connects to a "testing" API
-    npm run astro build -- --mode testing
+    npm run astro build --mode testing
     ```
  </Fragment>
  <Fragment slot="pnpm">