bindings: python: fix the default_val argument name in the doc string

bindings: python: fix the default_val argument name in the doc string

The argument in Line.request() is called default_val, not default_vals.
For backward compatibility with older versions default_vals would still
work if it was passed a tuple or list, but let's not confuse users:
default_val is the preferred way for single lines.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>