Fix small mistake in the Readme of kotlinx-coroutines-test (#1865)

There was a small mistake in the Readme of kotlinx-coroutines-test. A lateinit variable evaluated to null at the beginning.

kotlinx-coroutines-test/README.md

@@ -277,7 +277,7 @@ important to ensure that [cleanupTestCoroutines][TestCoroutineScope.cleanupTestC
 ```kotlin
 class TestClass {
     private val testScope = TestCoroutineScope()
-    private lateinit var subject: Subject = null 
+    private lateinit var subject: Subject
 
     @Before
     fun setup() {