[PyCon] Replace `r` by `rand` in every function examples

[bench]

When copy/pasting and execute samples from the documentation, we got errors like

invalid literal for int() with base 10: 'rrrrrrrr'

This origin of this issue was introduced in #145
as the term r was replaced with rand 2 month ago, as it was
interfering with people using one letter samples.

(details in commit d961313)

To fix the documentation, all occurrences of 'r' must be replaced by 'rand' like in the example below.

@swim
def pattern101(p=0.25, i=0):
    print(Pat('baba:r*8', i)) 
    again(pattern101, p=0.25, i=i+1)

should be updated with

@swim
def pattern101(p=0.25, i=0):
    print(Pat('baba:rand*8', i)) 
    again(pattern101, p=0.25, i=i+1)

And that's all folks.

[Bubobubobubobubo]

Done! Closing the issue :)