MINOR: [Docs] Update phrasing of intro sentence in the new contributors guide
MINOR: [Docs] Update phrasing of intro sentence in the new contributors guide

A tiny phrasing suggestion - I think this sounds stronger this way; it's not just **intended** to be a resource for new contributors, is **is** a resource for new contributors.

CC @AlenkaF - let me know what you think, happy to close this PR unmerged if the old phrasing is preferred!

Closes #13217 from thisisnic/new-contrib-language

Authored-by: Nicola Crane <b130ca97a5c118d9797a8d384d7c894a28655aef@gmail.com>
Signed-off-by: Nic Crane <b130ca97a5c118d9797a8d384d7c894a28655aef@gmail.com>

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