Fix typing in hexdump docstring

[justinsteven]

When calling hexdump() with a string-type s you get a BytesWarning:

>>> pwn.hexdump("abcd")
<stdin>:1: BytesWarning: Text is not bytes; assuming ASCII, no guarantees. See https://docs.pwntools.com/#bytes
'00000000  61 62 63 64                                         │abcd│\n00000004'

Problem being, IDEs get cranky if you call hexdump() with a bytes-type s due to the docstring.

Adjusted the docstring to indicate we expect bytes.

I'm sure there are many other instances of this worth tending to including the encoders (#1583, #1923 and so on) but this one in particular has been bugging me.

[justinsteven]

Can this please be given the label hacktoberfest-accepted. Thanks!