docs: run example fix
docs: run example fix

* Fixes example on how to run the world population analysis on the REANA cloud.

* Amends some verbiage to improve the reading flow.

Signed-off-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

reanahub/reana-demo-worldpopulation,tiborsimko/reana-demo-worldpopulation