[3.6] bpo-31537: Update readline documentation example. (GH-3925)

[miss-islington]

Change the code example from using get_history_length to get_current_history_length.
(cherry picked from commit eeb5ffd)

https://bugs.python.org/issue31537

[the-knights-who-say-ni]

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[miss-islington]

@infinitewarp and @Mariatta: Backport status check is done, and it's a success ✅ .

[miss-islington]

Thanks, @Mariatta!