Improve run_a_new_lister.rst file

Improvements needed: Remove extra space of the end of lines.

https://forge.softwareheritage.org/source/swh-lister/browse/master/docs/run_a_new_lister.rst$33 Add the exact location of conf/lister.yml file

https://forge.softwareheritage.org/source/swh-lister/browse/master/docs/run_a_new_lister.rst$50-51 Fix typos in this line

Related #1979 (moved)

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Migrated from T1946 (view on Phabricator)