Typo in §4.4

[systemed]

The spec currently says:

Feature attributes are encoded as pairs of integers in the tag field of a feature. [...] A feature MUST have an even number of tag fields. A feature tag field MUST NOT contain a key index or value index greater than or equal to the number of elements in the layer's keys or values set, respectively.

This should be tags, not tag.

I can do a quick PR to fix this but wanted to check first in case you thought it merited a 2.1.1 "bugfix" release (I don't think it does but YMMV).