[3.12] Update floatingpoint.rst (GH-110509) (#110513)

Update floatingpoint.rst (GH-110509)

This commit removes a ':'. I believe the extra colon causes a display error.

What I believe to be an error:
Above this expression
`round(math.pi, ndigits=2) == round(22 / 7, ndigits=2)`
the page displays `.. doctest::`.

What I observed:
After I remove the extra colon, the page does not display `.. doctest::`
(cherry picked from commit 8e56d551ceef37a307280bcc5303bf69ccc9f9c1)

Co-authored-by: zipperer <47086307+zipperer@users.noreply.github.com>