fix invalid printf formats

There are some format strings that are "invalid" in the pfff sources.

In general, invalid formats are nonsensical format strings that were accepted in OCaml 4.01.0 and earlier, but whose semantics is unspecified. They are still accepted by the new Printf implementation of OCaml 4.02.0 but in some cases with different semantics, and they will be statically rejected by OCaml 4.03.0.

You can check for them in OCaml 4.02.0 with the flag -strict-formats.

In the case of pfff,  I think the behavior has not changed between 4.01.0 and 4.02.0. You should still review the changes and make sure they correspond to the intended semantics.