Fix typo (#4181)

Author: lukevolpatti

doc/quick-overview.rst

@@ -48,7 +48,7 @@ Here are the key properties for a ``DataArray``:
 Indexing
 --------
 
-xarray supports four kind of indexing. Since we have assigned coordinate labels to the x dimension we can use label-based indexing along that dimension just like pandas. The four examples below all yield the same result (the value at `x=10`) but at varying levels of convenience and intuitiveness.
+xarray supports four kinds of indexing. Since we have assigned coordinate labels to the x dimension we can use label-based indexing along that dimension just like pandas. The four examples below all yield the same result (the value at `x=10`) but at varying levels of convenience and intuitiveness.
 
 .. ipython:: python