Fix gu binary name in matrix strategy example

[miguelaferreira]

This PR changes the name for the gu binaries in the matrix strategy example. At least for macOS the binary is gu, I guess that for linux it's the same and for windows gu.cmd.

[miguelaferreira]

I didn't realise the example was excluding options that don't match.

[DeLaGuardo]

exclude directive works a little bit weird.

This example

strategy:
  matrix:
    foo: [A, B, C]
    bar: [D, E]

will explode to this list of combinations:

[A D], [A E], [B D], [B E], [C D], [C E]

But if you add exclude like this:

strategy:
  matrix:
    foo: [A, B, C]
    bar: [D, E]
    exclude:
      - foo: C
        bar: D
      - foo: C
        bar: E

result will be this:

[A D], [A E], [B D], [B E]

so you are excluding explicitly combinations [CD] and [CE]

I tried to do the same excluding combination like

{os: linux-latest, gu-binary: gu.cmd}