Fixed typo in docs animation api

Found this doc typo thanks to this Stackoverflow question:
https://stackoverflow.com/questions/73002265/wrong-axes-limits/73002556

One of the example was mixing the pyplot interface with the object oriented
approach. This led to wrong axis limits.

Fixed the typo to use the object oriented approach.

Author: Davide-sd

doc/api/animation_api.rst

@@ -111,7 +111,7 @@ artist at a global scope and let Python sort things out.  For example ::
 
    fig, ax = plt.subplots()
    xdata, ydata = [], []
-   ln, = plt.plot([], [], 'ro')
+   ln, = ax.plot([], [], 'ro')
 
    def init():
        ax.set_xlim(0, 2*np.pi)