Merge pull request #1420 from 4geru/fix_rect_angle_name_in_structs_md

fix name from rectangle to Rectangle

Author: marioidival

src/custom_types/structs.md

@@ -86,7 +86,7 @@ fn main() {
 
 ### Activity
 
-1. Add a function `rect_area` which calculates the area of a rectangle (try
+1. Add a function `rect_area` which calculates the area of a `Rectangle` (try
    using nested destructuring).
 2. Add a function `square` which takes a `Point` and a `f32` as arguments, and
    returns a `Rectangle` with its lower left corner on the point, and a width and