Correct doc typos (atanpi2 -> atan2pi)

Author: jepler

Doc/library/math.rst

@@ -650,8 +650,8 @@ Trigonometric functions
    The vector in the plane from the origin to point ``(x, y)`` makes this angle
    with the positive X axis. The point of :func:`atan2pi` is that the signs of both
    inputs are known to it, so it can compute the correct quadrant for the angle.
-   For example, ``atanpi(1)`` and ``atanpi2(1, 1)`` are both ``1/4``, but
-   ``atanpi2(-1, -1)`` is ``-3/4``.
+   For example, ``atanpi(1)`` and ``atan2pi(1, 1)`` are both ``1/4``, but
+   ``atan2pi(-1, -1)`` is ``-3/4``.
 
 
 .. function:: cos(x)