Updated example for custom build targets in README

[stollr]

The example misses the level in the config.

massive_build:
    targets:
        main:
            target_one: ~
            target_two: ~
            target_three: ~
        quick:
            target_one: ~

Should be written as

massive_build:
    targets:
        main:
            dependencies:
                target_one: ~
                target_two: ~
                target_three: ~
        quick:
            dependencies:
                target_one: ~

[alexander-schranz]

@stollr Thank you!