bpo-37635: Update arg name for seek() in IO tutorial (GH-16147)

bpo-37635: Update arg name for seek() in IO tutorial (GH-16147)

Typically, the second positional argument for ``seek()`` is *whence*. That is the POSIX standard name (http://man7.org/linux/man-pages/man3/lseek.3p.html) and the name listed in the documentation for ``io`` module (https://docs.python.org/3/library/io.html#io.IOBase.seek).

The tutorial for IO is the only location where the second positional argument for ``seek()`` is referred to as *from_what*. I suspect this was created at an early point in Python's history, and was never updated (as this section predates the GitHub repository):

```
$ git grep "from_what"
Doc/tutorial/inputoutput.rst:To change the file object's position, use ``f.seek(offset, from_what)``.  The position is computed
Doc/tutorial/inputoutput.rst:the *from_what* argument.  A *from_what* value of 0 measures from the beginning
Doc/tutorial/inputoutput.rst:the reference point.  *from_what* can be omitted and defaults to 0, using the
```

For consistency, I am suggesting that the tutorial be updated to use the same argument name as the IO documentation and POSIX standard for ``seek()``, particularly since this is the only location where *from_what* is being used.

Note: In the POSIX standard, *whence* is technically the third positional argument, but the first argument *fildes* (file descriptor) is implicit in Python.

https://bugs.python.org/issue37635