Generalize the site URL

With such var env, the example is ready to use
everywhere, on any Gitlab installation.

doc/user/packages/maven_repository/index.md

@@ -707,23 +707,23 @@ You can create a new package each time the `master` branch is updated.
 
 1. Make sure your `pom.xml` file includes the following. 
    You can either let Maven use the CI environment variables, as shown in this example,
-   or you can hard code your project's ID.
+   or you can hard code your server's hostname and project's ID.
 
    ```xml
    <repositories>
      <repository>
        <id>gitlab-maven</id>
-       <url>https://gitlab.example.com/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
+       <url>${env.CI_SERVER_URL}/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
      </repository>
    </repositories>
    <distributionManagement>
      <repository>
        <id>gitlab-maven</id>
-       <url>https://gitlab.example.com/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
+       <url>${env.CI_SERVER_URL}/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
      </repository>
      <snapshotRepository>
        <id>gitlab-maven</id>
-       <url>https://gitlab.example.com/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
+       <url>${env.CI_SERVER_URL}/api/v4/projects/${env.CI_PROJECT_ID}/packages/maven</url>
      </snapshotRepository>
    </distributionManagement>
    ```