Fix a documentation for package.build option.
Fix a documentation for package.build option.

achanda/cargo,nipunn1313/cargo,nipunn1313/cargo,tshepang/cargo,vojtechkral/cargo,tsurai/cargo,rust-lang/cargo,rustyhorde/cargo,tshepang/cargo,rustyhorde/cargo,tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,tsurai/cargo,achanda/cargo,tsurai/cargo,achanda/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,achanda/cargo,vojtechkral/cargo,rustyhorde/cargo,eminence/cargo,tshepang/cargo,tsurai/cargo,rust-lang/cargo