Update floatingpoint.rst (python#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::`

Author: zipperer

Doc/tutorial/floatingpoint.rst

@@ -137,7 +137,7 @@ the :func:`math.isclose` function can be useful for comparing inexact values:
    True
 
 Alternatively, the :func:`round` function can be used to compare rough
-approximations::
+approximations:
 
 .. doctest::