Gate MVP example conflicts with BNF

[mkatychev]

The feature gate example lists a gate as having both a version and feature field: @since(version = 0.2.2, feature = fancy-foo)
https://github.com/WebAssembly/component-model/blob/main/design/mvp/WIT.md#feature-gates

However the BNF for gate syntax lists feature-field as only being used by an @unstable gate:

gate ::= gate-item*
gate-item ::= unstable-gate
            | since-gate
            | deprecated-gate

unstable-gate ::= '@unstable' '(' feature-field ')'
since-gate ::= '@since' '(' version-field ')'
deprecated-gate ::= '@deprecated' '(' version-field ')'

feature-field ::= 'feature' '=' id
version-field ::= 'version' '=' <valid semver>

[lukewagner]

(Sorry for the slow reply! I was out the last 2 weeks.). Thanks for the careful read and good catch! If I'm recalling history correctly, we started by allowing @since to have both a version and feature-name, and then removed the latter in #387, but we forgot to update this example. I'll fix the example shortly.