Update wfs-referencing.ipynb

change negative x-axis to x-axis

Author: fs446

doc/examples/wfs-referencing.ipynb

@@ -87,7 +87,7 @@
     "# reference contour is a straight line\n",
     "xref_line = 0\n",
     "# calc reference contour xref(x0), cf. [FFSS17, eq. (24), (31), (52)]\n",
-    "# this code assumes virtual point source on negative x-axis\n",
+    "# this code assumes virtual point source on x-axis\n",
     "cosbeta = (array.n @ [1, 0, 0]).reshape(-1, 1)\n",
     "xref = array.x + (xs - array.x) * \\\n",
     "    (xref_line + R * cosbeta) / (xs[0, 0] + R * cosbeta)\n",