FIX: LGTM error: Special method has incorrect signature

Too few parameters for special method __getitem__, which has 1 parameter, but should have 2, in class FileBasedImage.

https://lgtm.com/rules/7850093/

Author: DimitriPapadopoulos

nibabel/filebasedimages.py

@@ -202,7 +202,7 @@ def __init__(self, header=None, extra=None, file_map=None):
     def header(self):
         return self._header
 
-    def __getitem__(self):
+    def __getitem__(self, key):
         """ No slicing or dictionary interface for images
         """
         raise TypeError("Cannot slice image objects.")