Fix arity bug

[teytaud]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and Context / Related issue

@mathuvu found a bug.

How Has This Been Tested (if it applies)

import nevergrad as ng
from nevergrad.optimization import optimizerlib
arg = ng.p.TransitionChoice([0, 1], repetitions=5)
optim = optimizerlib._OnePlusOne(parametrization=arg, budget=6500, num_workers=1)
print(optim.arity_for_discrete_mutation)
optim = optimizerlib._OnePlusOne(parametrization=7, budget=6500, num_workers=1)
print(optim.arity_for_discrete_mutation)

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document and completed the CLA (see CLA).
• All tests passed, and additional code has been covered with new tests.

[jrapin]

simpler syntax