Update text in adf quantification find_flux_limits

Due to changes in find_flux_limits code: middle mouse button has
been changed to the Enter key, due to limitations with using the
middle mouse button in the matplotlib interface of Jupyter Notebooks.

adf_quantification_notebook/adf_quantification.ipynb

@@ -84,9 +84,9 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Select the left limit with the left mouse button, the right limit with the right mouse button and then confirm with the centre button. Good values here could be 103 for the left limit, and 406 for the right limit.\n",
+    "Select the left limit with the left mouse button, the right limit with the right mouse button and then confirm with the Enter key. Good values here could be 103 for the left limit, and 406 for the right limit.\n",
     "\n",
-    "Once coordinates have been seleted a message will be printed above confirming.\n",
+    "Once coordinates have been selected a message will be printed above confirming.\n",
     "\n",
     "The limits can also be given directly to the function:"
    ]