Fix link to the ThreeDCoordinate section

Author: PeterJCLaw

programming/sr/vision/index.md

@@ -157,7 +157,7 @@ spherical
 :   A [`Spherical`](#Spherical) instance describing the position relative to the camera.
 
 cartesian
-:   A [`ThreeDCoordinates`][#ThreeDCoordinates] instance describing the absolute position of the marker relative to the camera.
+:   A [`ThreeDCoordinate`](#ThreeDCoordinate) instance describing the absolute position of the marker relative to the camera.
 
 [`Coordinate`](#Coordinate) {#Coordinate}
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