Adding an example for PRump.

[grouptheoryenthusiast]

Addressing issue #1832.
Added a simple example to the description of the PRump function to demonstrate how to call it.

[olexandr-konovalov]

@grouptheoryenthusiast thank you! The test of this PR failed in https://travis-ci.org/gap-system/gap/jobs/343088351 because the example is likely sensitive to some randomised algorithms or the set of packages loaded:

########> Diff in /home/travis/build/gap-system/gap/doc/ref/../../lib/grp.gd:
3211 (/home/travis/build/gap-system/gap/tst/testmanuals/chapter39.tst:375)
# Input is:
PRump(g,2);
# Expected output:
Group([ (), (2,3,4), (1,2,3), (1,3)(2,4) ])
# But found:
Group([ (), (1,3,2), (1,3)(2,4), (1,4,3) ])
########
chapter39.tst

A simple way to resolve this is to update example to use the output below. But then in the future this will likely require amending again. I think it may be better to make an example which demonstrates some property of the output, e.g. that it is equal to G' G^p.

[markuspf]

Could you squash the two commits into one? (Let me know if you need help with the git magic)

[grouptheoryenthusiast]

Hi Markus,
I have squashed the two commit messages into one using rebase, and then I pushed the branch using: git push origin +fix/master/prump .
I am hoping that this is correct, I am still quite new to the finer details of git.
Please let me know if there is anything else I should do here.

[markuspf]

All fine. Thanks!