doc: Correct example build target names

Author: hodlinator

doc/usage.md

@@ -19,6 +19,6 @@ A simple interface description can be found at [test/mp/test/foo.capnp](../test/
 A more complete example can be found in [example](../example/) and run with:
 
 ```sh
-make -C build example
+make -C build mpexample mpprinter mpcalculator
 build/example/mpexample
 ```