Corrected some descriptive text.

tools/Jupyter notebooks/plot_source_wave.ipynb

@@ -192,7 +192,7 @@
     "print('Minimum wavelength: {:g} m'.format(wmin))\n",
     "print('Maximum spatial resolution: {:g} m'.format(dmin))\n",
     "print('Time step for 3D cubic cell: {:g} s'.format(dt3D))\n",
-    "print('Time step for 2D cubic cell: {:g} s'.format(dt2D))"
+    "print('Time step for 2D square cell: {:g} s'.format(dt2D))"
    ]
   }
  ],