update index definition in the "common principles" section.

[sappelhoff]

@nicholst I just realized that we probably missed a definition in the "common principles" section.

See: https://bids-specification.readthedocs.io/en/latest/02-common-principles.html#definitions

under point 9:

<index> - a numeric value, possibly prefixed with arbitrary number of 0s for consistent indentation, e.g., it is 01 in run-01 following run- specification.

should this become a nonnegative numeric value ? cc @tsalo @effigies

Originally posted by @sappelhoff in #578 (comment)