pointing out that a trailing ';' is the same as NULL

docs/examples/notebooks/rmagic_extension.ipynb

@@ -61,7 +61,7 b''
        "output_type": "pyout",
        "prompt_number": 2,
        "text": [
-        "<matplotlib.collections.PathCollection at 0x10d7b5650>"
+        "<matplotlib.collections.PathCollection at 0x10f32f610>"
        ]
       }
      ],
@@ -426,7 +426,7 b''
     {
      "cell_type": "markdown",
      "source": [
-      "The return value can be suppressed by adding NULL as the lart line, or an -n argument.",
+      "The return value can be suppressed by a trailing ';' or an -n argument.",
       ""
      ]
     },
@@ -444,7 +444,7 b''
      "cell_type": "code",
      "collapsed": true,
      "input": [
-      "%R X; NULL"
+      "%R X; "
      ],
      "language": "python",
      "outputs": [],