fix: TypeScript declaration return type of Acl.open()

[harmony7]

This is a quick fix of the types declaration fix acl.d.ts to fix this error

% npm run build

> prebuild
> tsc

node_modules/@fastly/js-compute/types/acl.d.ts:8:12 - error TS7010: 'open', which lacks return-type annotation, implicitly has an 'any' return type.

8     static open(aclName: string);
             ~~~~


Found 1 error in node_modules/@fastly/js-compute/types/acl.d.ts:8