Remove redundant verb (#252)

There should only be one verb.

Author: felixcremer

overview/xarray-in-45-min.ipynb

@@ -8,7 +8,7 @@
     "\n",
     "# Xarray in 45 minutes\n",
     "\n",
-    "In this lesson, we discuss cover the basics of Xarray data structures. By the\n",
+    "In this lesson, we cover the basics of Xarray data structures. By the\n",
     "end of the lesson, we will be able to:\n",
     "\n",
     "- Understand the basic data structures in Xarray\n",