First major update of src/2_coordinates.md

src/2_coordinates.md

@@ -21,7 +21,7 @@ The most common coordinates are *Cartesian coordinates*, where we use a
 number $n$ of perpendicular axes. The coordinates corresponding to these
 axes are $x_j$ where $j=1, \ldots, n$.
 
-Cartesian coordinates are simple to describe and operate in. The coordinate axes are 
+Cartesian coordinates are simple to describe and operate on. The coordinate axes are 
 straight lines perpendicular to each other. It is therefore
 very easy to do calculations in Cartesian coordinates. For example,
 the distance $\Delta s$ between two points $(x_1, x_2, \ldots, x_n)$