Chapter name change

gradle/subprojects.gradle

@@ -14,7 +14,7 @@ project(':understandingcollections') {
     }
 }
 
-project(':threads') {
+project(':concurrent') {
     dependencies {
         compile project(':enums')
     }
@@ -67,4 +67,4 @@ configure(subprojects - project(':onjava')) {
         compile 'com.google.guava:guava:19.0'
         compileOnly "org.openjdk.jmh:jmh-core:${jmh.jmhVersion}"
     }
-}
+}