fix: square brackets (#21698)

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## Description

I updated the heading to replace <project directory> with the
HTML-escaped equivalent &lt;project directory&gt;. This change ensures
proper rendering on the web version of the site, where raw HTML don't
display correctly..

## Reviews

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- [ ] Editorial review

Co-authored-by: Sergii Boboshko <sergii.boboshko@henesis.eu>

Author: b-c-v

content/manuals/compose/how-tos/environment-variables/variable-interpolation.md

@@ -203,7 +203,7 @@ $ docker compose --env-file ./config/.env.dev up
   $ docker compose --env-file .env.dev up -e DATABASE_URL=mysql://new_user:new_password@new_db:3306/new_database
   ```
 
-### local `.env` file versus <project directory> `.env` file
+### local `.env` file versus &lt;project directory&gt; `.env` file
 
 An `.env` file can also be used to declare [pre-defined environment variables](envvars.md) used to control Compose behavior and files to be loaded.