Fix non convergence in example of usage (#13)

Hi,

The current example of usage does not converge in the amount of
iterations specified

```python
import netket as nk
import netket_fidelity as nkf

# Create the Hilbert space and the variational states |ψ⟩ and |ϕ⟩
hi = nk.hilbert.Spin(0.5, 4)
sampler = nk.sampler.MetropolisLocal(hilbert=hi, n_chains_per_rank=16)
model = nk.models.RBM(alpha=1, param_dtype=complex, use_visible_bias=False)
phi = nk.vqs.MCState(sampler=sampler, model=model, n_samples=100)
psi = nk.vqs.MCState(sampler=sampler, model=model, n_samples=100)

# Transformation U
U = nkf.operator.Hadamard(hi, 0)

# Create the driver
optimizer = nk.optimizer.Sgd(learning_rate=0.01)
te =  nkf.driver.InfidelityOptimizer(phi, optimizer, U=U, U_dagger=U, variational_state=psi, is_unitary=True, cv_coeff=-1/2)

# Run the driver
te.run(n_iter=100)
```

This keeps yielding an infidelity around 0.5. Either adding more
iterations or changing to the Adam optimizer fixes this.
With Adam and the same number of iterations, infidelity is consistently
smaller than 0.001

Co-authored-by: Mohammed Boky <mohammedboky@gmail.com>

README.md

@@ -61,7 +61,7 @@ psi = nk.vqs.MCState(sampler=sampler, model=model, n_samples=100)
 U = nkf.operator.Hadamard(hi, 0)
 
 # Create the driver
-optimizer = nk.optimizer.Sgd(learning_rate=0.01)
+optimizer = nk.optimizer.Adam(learning_rate=0.01)
 te = nkf.driver.InfidelityOptimizer(phi, optimizer, U=U, U_dagger=U, variational_state=psi, is_unitary=True, cv_coeff=-1/2)
 
 # Run the driver