fix: switch it to a defined variable. 04_Null Safety.md (Kotlin#55)

notNull argument on line 26 is not defined. Switch it to a defined variable.

Author: chrismontone

examples/01_introduction/04_Null Safety.md

@@ -23,7 +23,7 @@ fun main() {
         return notNull.length
     }
 
-    strLength(notNull)                                      // 8
+    strLength(neverNull)                                    // 8
     strLength(nullable)                                     // 9
 //sampleEnd
 }