adding commas

Co-authored-by: soranjh <40344468+soranjh@users.noreply.github.com>

pennylane/fermi/fermionic.py

@@ -329,7 +329,7 @@ def shift_operator(self, initial_position, final_position):
         There are three `anti-commutator relations <https://en.wikipedia.org/wiki/Creation_and_annihilation_operators#Creation_and_annihilation_operators_in_quantum_field_theories>`_:
 
         .. math::
-            \left\{ a_i, a_j \right\} = 0 \quad \left\{ a^{\dagger}_i, a^{\dagger}_j \right\} = 0 \quad \left\{ a_i, a^{\dagger}_j \right\} = \delta_{ij}
+            \left\{ a_i, a_j \right\} = 0, \quad \left\{ a^{\dagger}_i, a^{\dagger}_j \right\} = 0, \quad \left\{ a_i, a^{\dagger}_j \right\} = \delta_{ij},
 
 
         where