Update preview builds instructions for Yarn v1 (mobile)

[mcmire]

On mobile, the instructions to integrate a preview build into the client are slightly different than on extension. Instead of e.g.

"@metamask/controller-utils@^1.1.4": "npm:@metamask-previews/[email protected]"

you must say (note the lack of the version specifier on the left-hand side):

"@metamask/controller-utils": "npm:@metamask-previews/[email protected]"

Acceptance Criteria

• The preview builds section of the Contributing doc contains instructions for Yarn v1 projects