Corrected a typo

mbtrack2/tracking/beam_ion_effects.py

@@ -706,7 +706,7 @@ class BeamIonElement(Element):
                                               sb_stdx, sb_stdy, sb_mx, sb_my)
 
         elif field_model == "PIC":
-            if isinstance(second_beam, IonParticles):
+            if isinstance(first_beam, IonParticles):
                 fb_mx, fb_my, fb_stdx, fb_stdy = first_beam.mean_std_xy
             else:
                 fb_stdx = first_beam["x"].std()