Wrong command line flag used for initial corpus directory

[cailloumajor]

Hello,

If I understand correctly, cargo hfuzz passes the following argument to hongfuzz binary:

honggfuzz-rs/src/bin/cargo-hfuzz.rs

Lines 176 to 177 in dc32d19

	"-f",
	&honggfuzz_input,

But I can't find the -f flag anywhere in https://github.com/google/honggfuzz/blob/348a47213919f14b9453e89a663b1515369bd9a2/docs/USAGE.md. In the doc, the flag used to indicate the path to the initial input corpus directory is -i.

Moreover, IMO a default value being the input sub-directory in the fuzzing workspace should not be provided, because hongfuzz already uses it for its generated input corpus files, so it is not suitable for user-provided files.

I would be happy to submit a PR if needed.