BIP 21: Add a missing "&" rule to the ABNF grammar

The URI syntax uses an ampersand to separate query parameters,
but the ABNF grammar didn't reflect this fact.

Also, the type of grammar used here is "ABNF" and not "BNF".
BIP 21: Mention RFC 3986 and character set
BIP 21: Precisely define the valid query characters
BIP 72: Reduce the amount of escaping needed

RFC 3986 obsoletes RFC 1738, which this BIP was wrongly referencing.
The new RFC requires far less escaping for query parameters.