pxt-microbit Accessibility PR (#529)

* Accessibility changes

docs/courses/csintro/coordinates/overview.md

@@ -2,21 +2,21 @@
 
 Through math class, most middle school students are already familiar with coordinate grids and mapping x and y coordinates on a plane. To review some terms:
 
-#### Axes
+## Axes
 * The basic coordinate grid a student learns has two axes,
 
 >* an x-axis which runs horizontally and 
 * a y-axis which runs vertically.
 	
-#### Origin
+## Origin
 * These two axes meet at a point called the origin where both the x and the y values are zero.
 * On this basic coordinate grid, the origin is in the lower left corner of the grid and has the coordinates (0,0).
 	
-#### Coordinate pair
+## Coordinate pair
 * The first value in a coordinate pair is the x value and the second value in a coordinate pair is the y value.
 * A simple way to remember which value comes first is to remember their order in the alphabet. The letter x comes before the letter y in the alphabet and the x coordinate comes before the y coordinate in a coordinate pair.
 	
-#### Coordinate value changes
+## Coordinate value changes
 * On a basic coordinate grid,
 
 >* the value of the x coordinate increases left to right and is a measure of how many units a point is horizontally from the origin