ABNF: add reference to RFC errata
ABNF: add reference to RFC errata

Hex digits are case-insensitive according to the errata. It is apparently a common misunderstanding that the RFC only allows upper-case hex digits.
antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4,antlr/grammars-v4