Usage: use 'implementation' instead of 'api' in Gradle

usage.html

@@ -147,7 +147,7 @@ <h3 class="subtema" id="gradle">Usage from a Gradle project</h3>
 Then, in your <code>build.gradle</code> file:
 </p>
 <pre class="code"><code class="language-groovy">dependencies {
-    api "io.sf.carte:css4j:${css4jVersion}"
+    implementation "io.sf.carte:css4j:${css4jVersion}"
 }
 </code></pre>
 <p>