docs(piscine-rust) Add missing "s"

subjects/highest/README.md

@@ -5,7 +5,7 @@
 In this exercise, a `Numbers` struct will be given.
 
 These methods have to be written for it:
-- `new`: create a new instance of `Number`.
+- `new`: create a new instance of `Numbers`.
 - `List`: which returns an `array` with every number in the struct.
 - `Latest`: which returns an `Option<u32>` with the last added number.
 - `Highest`: which returns an `Option<u32>` with the highest number from the list.