Merge #202: doc: correct the build instructions for the example

b6d3dc4 doc: correct the build instructions for example (Novo)

Pull request description:

  The make target to build the example executable is "mpexample", not "example", so `make example` does nothing. This PR changes the documentation to use "mpexamples", because that builds the calculator, printer and example executables.

ACKs for top commit:
  Sjors:
    ACK b6d3dc4
  ismaelsadeeq:
    ACK b6d3dc4
  ryanofsky:
    Code review ACK b6d3dc4. Thanks for the fix!

Tree-SHA512: 7573537915b7cccf6cf4f951abf4216eebddb39383ccadd06a516cd71821bb7b657b3a0d465ecf2673a6006f3b88f7ee60b7bc34fe6ccf27b17a846b43293c72

Author: ryanofsky

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
 ```