mainClassName is wrong in client-tools

Without the fix, ./gradlew client-tools:run fails with:

> Task :client-tools:run FAILED
Error: Could not find or load main class io.ktor.samples.clientmultipart.MultipartAppKt
Caused by: java.lang.ClassNotFoundException: io.ktor.samples.clientmultipart.MultipartAppKt

generic/samples/client-tools/build.gradle

@@ -10,7 +10,7 @@ buildscript {
 apply plugin: 'kotlin'
 apply plugin: 'application'
 
-mainClassName = "io.ktor.samples.clientmultipart.MultipartAppKt"
+mainClassName = "ToolsAppKt"
 
 sourceSets {
     main.kotlin.srcDirs = [ 'src' ]