Corrected typo in function name and doctests. rkf45.py (#10518)

* Corrected typo in function name and doctests. rkf45.py

There was a mistake in name of function (runge_futta_fehlberg instead of runge_kutta_fehlberg) . I have corrected this in function name and  also doctest.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Rename rkf45.py to runge_kutta_fehlberg_45.py

* Update runge_kutta_fehlberg_45.py

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <[email protected]>