Fix animation filename in introduction.rst
Fix animation filename in introduction.rst

It seems `bqplot-screencast.gif` was renamed `wealth-of-nations.gif`
but the reference to the filename was not updated here.

See:
- https://github.com/bloomberg/bqplot/search?q=screencast&type=Commits
- https://github.com/bloomberg/bqplot/commits/057c51e6f657b8e78cf3694785ab4b7c3a25c4f5/bqplot-screencast.gif
- https://github.com/bloomberg/bqplot/blob/057c51e6f657b8e78cf3694785ab4b7c3a25c4f5/bqplot-screencast.gif
SylvainCorlay/bqplot,dmadeka/bqplot,bloomberg/bqplot,bloomberg/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,dmadeka/bqplot,ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot