Disambiguate the use of the term value in the example.

[metaperl]

The basic example in the docs is confusing because an attribute is named value (one use of value in the example) and the dictionary with the proposed-new-value has a key named value.

Because the dictionary key cannot be easily changed, I renamed the class attribute to data to disambiguate it.

[Carreau]

LGTM, failure unrelated. Thanks for taking the time !