doc: correct the build instructions for example

The cmake target to build the example executable is "mpexample" not "example".
This commit uses "mpexamples" in the doc, because that builds
the calculator, printer and example executables.

Author: Eunovo

doc/usage.md

@@ -19,6 +19,9 @@ 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
-build/example/mpexample
+mkdir build
+cd build
+cmake ..
+make mpexamples
+example/mpexample
 ```