[3.12] gh-125383: Update `fib` function comment for accuracy (GH-125386) (#125396)

gh-125383: Update `fib` function comment for accuracy (GH-125386)

`Doc/tutorial/controlflow.rst`: fix comment for `fib` function
(cherry picked from commit 283ea5f3b2b6a18605b8598a979afe263b0f21ce)

Co-authored-by: Wulian <xiguawulian@gmail.com>