Fix mixin data merging documentation

[vetruvet]

The current documentation states that data defined by a mixin gets merged with the component's data using a shallow merge. This is not true - the merge is actually a recursive one. This is a small wording change so that the documentation correctly describes current behavior.

This behavior can be easily seen in the source for the function that merges data (in optionMergeStrategies): https://github.com/vuejs/vue/blob/master/src/core/util/options.js#L48. The function will recursively merge keys from both objects.