Here's why:
* Linking verbs connect the subject of a sentence to a word or phrase that renames or describes the subject.
* In this case, "raisin" (the subject) is linked to the phrase "a dried grape" which describes what a raisin is.
Helping verbs are used in combination with another verb to create a verb phrase (e.g., "is eating", "will have been seen"). There's no other verb involved in this sentence, so "is" isn't a helping verb.