tf2 listener cpp tutorial: minor syntax/wording improvements (backport #4434)

[mergify]

some changes i think might improve readability, as discovered in osrf/ros2_test_cases#1176.
i would have liked to add a link to a more generic "tf concepts" page, but i didnt find anything satisfactory. i really like tho the first two paragraphs in #4235 which i found very insightful especially around the question of target/source terminology and frame order.

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This is an automatic backport of pull request #4434 done by Mergify.

[mergify]

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	both modified:   source/Tutorials/Intermediate/Tf2/Writing-A-Tf2-Listener-Cpp.rst

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