document getindex() with range of length 1. (JuliaLang#41140)

Added an example with A[:, 3:3] which returns a Matrix instead of a
Vector. Somebody made me aware of this behavior on
[stackoverflow](https://stackoverflow.com/questions/67889148/let-indexing-return-a-matrix-instead-of-a-vector-in-julia).
but I couldn't find an example of it in the documentation.

doc/src/manual/arrays.md

@@ -695,6 +695,12 @@ julia> A[:, 3]
  13
  15
  17
+
+julia> A[:, 3:3]
+3×1 Matrix{Int64}:
+ 13
+ 15
+ 17
 ```
 
 ### Cartesian indices