Update GenericContainer Usage section instead of the one previously…

… updated

README.md

@@ -181,13 +181,13 @@ The most flexible but less convenient container type is `GenericContainer`. This
 with custom configuration.
 
 ```scala
-class GenericContainerSpec extends AnyFlatSpec with TestContainerForAll {
-  override val containerDef = GenericContainer.Def("nginx:latest",
+class GenericContainerSpec extends AnyFlatSpec with ForAllTestContainer {
+  override val container: GenericContainer = GenericContainer("nginx:latest",
     exposedPorts = Seq(80),
     waitStrategy = Wait.forHttp("/")
   )
 
-  "GenericContainer" should "start nginx and expose 80 port" in withContainers { case container =>
+  "GenericContainer" should "start nginx and expose 80 port" in {
     assert(Source.fromInputStream(
       new URL(s"http://${container.containerIpAddress}:${container.mappedPort(80)}/").openConnection().getInputStream
     ).mkString.contains("If you see this page, the nginx web server is successfully installed"))
@@ -455,6 +455,15 @@ object NginxContainer {
 }
 ```
 
+However, if you don't want to create a custom container, you can use `GenericContainer` directly while overriding `containerDef`:
+
+```scala
+override val containerDef = GenericContainer.Def("nginx:latest",
+  exposedPorts = Seq(80),
+  waitStrategy = Wait.forHttp("/")
+)
+```
+
 ### Migration from the classic API
 
 1. If you have custom containers created with the `GenericContainer`, add `ContainerDef` in the companion like this: