Incorrect spelling, explanation: ammended is a mistake because the base verb here is amend, coming from Middle English amenden – with only one consonant m. The past form of this verb is amended (as the verb is regular). This means that the correct form, in this case, is amended.

Correct spelling, explanation: the word comes from Middle English amenden and this spelling transformed a little but only got shorter without changing other letters, thus the correct modern form is amend (past form: amended). There is no need to add another m, so ammended is an incorrect form, and the only correct one is amended.

Definition of amended:
verb, past form of amend to modify or add something, for example, information or rules
The tax law is often amended.
The tariff for electricity was amended and now we pay less.