Fix column computation in documentation

[alberth]

The column computation function in the documentation had a few problems:

1. Comment spoke about input parameter, but it's actually text.
2. Something off-by-one must be happening, as last_cr ends up at the newline character, except at the first line.

With respect to the second item, I hacked a quick test (attached), which also incudes the fixed version.

I tried to make the function as small as possible so I deleted the if statement, so the function now relies on rfind returning -1 if it failed to find a new-line. Not sure if you like that, however it is documented functionality: https://docs.python.org/3/library/stdtypes.html?highlight=str%20rfind#str.rfind so should be quite stable behaviour.

t.py.txt