rev-parse doc: pass "--" to rev-parse in the --prefix example

rev-parse doc: pass "--" to rev-parse in the --prefix example

The "--" argument avoids "ambiguous argument: unknown revision or
path not in the working tree" errors when a pathname argument refers
to a non-existent file.

The "--" passed explicitly to set was removed because rev-parse
outputs the "--" argument that it is given.

Signed-off-by: Richard Hansen <redacted>
Signed-off-by: Junio C Hamano <redacted>