Update fake_file.py

fix comment typo

Author: davidlbaird

pyfakefs/fake_file.py

@@ -999,7 +999,7 @@ def readable(self) -> bool:
         return self.open_modes.can_read
 
     def writable(self) -> bool:
-        """Returns the readable state of the file."""
+        """Returns the writable state of the file."""
         return self.open_modes.can_write
 
     def _sync_io(self) -> None: