Clarify the prefix used for `extras` in TZIP-16
TZIP-012 and TZIP-016 provide clashing examples of how to prefix extras
in the interface
field.
We've decided to go with a -
vs a space, and I've made that change here. After this PR, TZIP-012 and TZIP-016 now align in their implementation description of the interfaces
field.
Discussion and Context available at: https://tezos-dev.slack.com/archives/C0129LBH6F3/p1613029785087000