[MINOR] Use <= for clarity in Pi examples' Monte Carlo process

## What changes were proposed in this pull request?

If my understanding is correct we should be rather looking at closed disk than the opened one.
## How was this patch tested?

Run simple comparison, of the mean squared error of approaches with closed and opened disk.
https://gist.github.com/mrydzy/1cf0e5c316ef9d6fbd91426b91f1969f
The closed one performed slightly better, but the tested sample wasn't too big, so I rely mostly on the algorithm  understanding.

Author: Maria Rydzy <majrydzy+gh@gmail.com>

Closes #15687 from mrydzy/master.