Add doc for ST_ISOVist v4

docs/dev/ST_ISOVist.md

@@ -30,7 +30,7 @@ Resulting polygon will be enclosed by a circle defined by the maximum distance (
 
 * `point` : Point coordinates (x, y, (z)) from which the visibility will be calculated,
 * `obstacles` : Geometry or set of geometries grouped into a simple or complex geometry, used as obstacles,
-* `maxDistance` : Only obstacles located within this distance from the `point` are considered in the calculation *(only cartesian distance are accepted - depends on your spatial ref)*,
+* `maxDistance` : Only obstacles located within this distance from the `point` are considered in the calculation *(only cartesian distance is accepted - depends on your spatial ref)*,
 * `angleStart` : Starting angle from which visibility will be calculated *(exprimed in radian - to convert into degree, use the H2 database [PI()](http://h2database.com/html/functions.html#pi) function)*,
 * `angleStop` : Ending angle from which visibility will be calculated *(exprimed in radian)*.