[spec] Outdated statements about glyph classes

[khaledhosny]

From 2.g.i:

A range of glyphs is denoted by a hyphen:

[<firstGlyph> - <lastGlyph>]

Spaces around the hyphen are not required since hyphens are not permitted in feature file glyph names. For example:

[\0-\31]
[A-Z]

But I believe this is no longer the case since hyphen in glyph names are permitted now.

[iterumllc]

I updated this language.