README: use the assignment operator in the Gradle example

Author: carlosame

README.md

@@ -65,7 +65,7 @@ project's own Maven repository in a `repositories` section of your build file:
 ```groovy
 repositories {
     maven {
-        url "https://css4j.github.io/maven/"
+        url = "https://css4j.github.io/maven/"
         mavenContent {
             releasesOnly()
         }