docs: fix --use-xpathhelper usage

[miguelbf-alb]

Pyangbind --use-xpath-helper is incorrectly mentioned in the usage documentation. The actual option is --use-xpathhelper. See implementation:

pyangbind/pyangbind/plugin/pybind.py

Line 228 in 88db69c

"--use-xpathhelper",

Also in xpathhelper docs the option is correctly named:

pyangbind/docs/xpathhelper.md

Line 62 in 88db69c

To initialise a YANGPathHelper class and use it with PyangBind-generated classes, the bindings must have been specified with the `--use-xpathhelper` argument. This ensures that the bindings are configured to pass the `path_helper` reference to one another as new classes are instantiated.

[JoseIgnacioTamayo]

LGTM, thanks for the documentation fix.

For some other reason the Unit Tests are failing, I will see to fix those tests and then this MR will go through.

[JoseIgnacioTamayo]

Could you please rebase to the lastest master, so the unit tests pass?

[miguelbf-alb]

@JoseIgnacioTamayo done, thanks