fix invalid printf formats

There are some format strings that are "invalid" in the spdiff 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 spdiff, I don't think there is any change of semantics between 4.01.0 and 4.02.0 but you should review the formats carefully to make sure they are really what's intended.