It is a "mistake" or a "slip of the tongue". It can also be called a "Malapropism" from the literary character Mrs Malaprop, (from Shreridan's The Rivals) who frequently made such errors in ways that were amusing to the readers. That term is reserved for the case where the thing that is actually said is a real word, but an obviously wrong one. Your example is such a case.
The Wikipedia article linked to defines "Malapropism" as:
An instance of speech error is called a malapropism when a word is produced which is nonsensical or ludicrous in context, yet similar in sound to what was intended.