fix doc typo

doc/POINTS_COORDINATES.md

@@ -40,7 +40,7 @@ Lastly, these is a system called **segment** (seg) coordinates.  These are only
 used in saving/loading submaps and you are unlikely to encounter them.
 
 As well as absolute and local coordinates, sometimes we need to use coordinates
-relative so some larger scale.  For example, when performing mapgen for a
+relative to some larger scale.  For example, when performing mapgen for a
 single overmap, we want to work with coordinates within that overmap.  This
 will be an overmap terrain-scale point relative to the corner of its containing
 overmap, and so typically take `x` and `y` values in the range [0,180).