Fix inverted logic in check_disallowed_field (#19)

Previously, the logic of the `verification.check_disallowed_field`
method was inverted. Instead of returning a violation when the
disallowed field was present, it was returning the violation when it
was not present. My suspicion is that this was due to a copy/paste
error.

This fixes this bug, by returning a violation only when a disallowed
field is present.
The bug was first found in the logic of the linter rule of:
`ieeetran_rules.check_in_collection`.

Fixes #18

Author: s-heppner

bibtex_linter/verification.py

@@ -85,7 +85,7 @@ def check_disallowed_field(entry: BibTeXEntry, field: str, explanation: str) ->
     If it does exist, include the explanation sentence in the invariant violation text to help the user understand
     why it is disallowed.
     """
-    if field not in entry.fields.keys():
+    if field in entry.fields.keys():
         return [f"Entry '{entry.name}' contains disallowed field [{field}]. {explanation}"]
     return []