Correct the Wormhole burn address generation description in spec

[imaginaryusername]

Per the genburn code at https://github.com/copernet/genburn/blob/master/cashaddr_test.go, the criteria for burn address selection seems to be that it must end with "8whc" preceded by 0-9, a stricter criteria than described in the spec (https://github.com/copernet/spec/blob/master/wormhole-burn-addr.pdf) where only "ending in whc" is required. The final generated wormhole burn address agrees with the code instead of the spec. Please fix the difference.