Update _gfm_observer.py

Author: mhinkkan

motulator/grid/control/_gfm_observer.py

@@ -127,11 +127,11 @@ class ObserverBasedGridFormingController:
         Active resistance (Ω), defaults to `0.25*u_nom/i_max`.
     k_v : float, optional
         Voltage control gain, defaults to `alpha_o/w_nom`.
-    alpha_o : float
+    alpha_o : float, optional
         Observer gain (rad/s), defaults to 2*pi*50.
     alpha_c : float, optional
         Current control bandwidth (rad/s), defaults to 2*pi*400.
-    u_nom : float
+    u_nom : float, optional
         Nominal grid voltage (V), line-to-neutral peak value, defaults to
         `sqrt(2/3)*400`.
     T_s : float, optional