Fix example and language in manpage

[mb720]

Hi and thanks a lot for rmlint!

I'd like to offer some improvements for the manpage.

It's mostly typo fixes and attempts to make the manpage easier to read.

But I think I also spotted an error in the examples. In the old version, rmlint -e was described as

Find duplicates with same basename (excluding extension)

Yet I'm pretty sure that option -b does that, whereas -e makes rmlint consider two files as duplicates only if their extensions match.