docs: update return type of get_molecular_hamiltonian (#1166)

Found a type error in the
[docs](https://quantumai.google/reference/python/openfermion/chem/MolecularData#get_molecular_hamiltonian)
so I figured I fix it.

I have directly opened a PR as I felt opening an issue was not the way
to go. Please tell me if this was the right way (first PR here) !

Related to #120

Signed-off-by: baptiste.bellot <baptiste.bellot@epita.fr>
Co-authored-by: Michael Hucka <mhucka@google.com>

Author: BapisteB

src/openfermion/chem/molecular_data.py

@@ -1010,7 +1010,7 @@ def get_molecular_hamiltonian(self, occupied_indices=None, active_indices=None):
                 which orbitals should be considered active.
 
         Returns:
-            molecular_hamiltonian: An instance of the MolecularOperator class.
+            molecular_hamiltonian: An instance of the InteractionOperator class.
 
         Note:
             The indexing convention used is that even indices correspond to