Fix examples to return 0 on success (#9)

example/hello/hello.cpp

@@ -10,6 +10,5 @@
 int main()
 {
     std::cout << "Hello!\n";
-    return 1;
 }
 // end::source[]

example/time/hello.cpp

@@ -9,5 +9,4 @@
 int main()
 {
     std::cout << "Hello!\n";
-    return 1;
 }