Fixed some typos

I think it should be "Permutation". Look at the example below this (reproduced here)

```py
     >>> p.rank()
     6
     >>> p + 1
     Permutation([1, 0, 3, 2])
     >>> p.next_lex()
     Permutation([1, 0, 3, 2])
     >>> _.rank()
     7
```

What it is trying to say is that adding a number to a Permutation gives the permutation with that many more rank.  Here adding 1 to the permutation of rank 6 gives the permutation of rank 7. "Rank" is an unfortunate term here IMHO. "Ordering" would be better (but I suspect this is an issue for the mathematicians who coined the term).