update !pip to %pip per python-control#1113

Author: murrayrm

examples/python-control_tutorial.ipynb

@@ -47,7 +47,7 @@
     "    import control as ct\n",
     "    print(\"python-control\", ct.__version__)\n",
     "except ImportError:\n",
-    "    !pip install control\n",
+    "    %pip install control\n",
     "    import control as ct"
    ]
   },
@@ -64,7 +64,7 @@
     "    \n",
     "If you are using [Google Colab](https://colab.research.google.com), use the following command at the top of the notebook to install the `control` package:\n",
     "\n",
-    "    !pip install control\n",
+    "    %pip install control\n",
     "\n",
     "(The import code above automatically runs this command if needed.)\n",
     "    \n",