Documentation: Refresh pygments dependency version for GAS notation fix

[btashton]

The PR I submitted to pygments for fixing the rendering of intimidates in GAS ARM assembly is now part of the 2.7.0 release, so we should refresh the dependencies.
https://github.com/pygments/pygments/blob/3e1b79c82d2df318f63f24984d875fd2a3400808/CHANGES#L62

https://github.com/apache/incubator-nuttx/blob/a49a3eae51ab48c11d11e02215ab2a6cff7e9a98/Documentation/Pipfile.lock#L148

This is mostly to remind me to do it, but if someone else wants to go for it please do. It should be as simple as running pipenv lock and making sure the CI passes.