docs(rpn): make error handling bullet-points

subjects/rpn/README.md

@@ -6,10 +6,11 @@ Write a **program** which takes a `string` containing an equation written in `Re
 
 `Reverse Polish Notation` is a mathematical notation in which every operator follows all of its operands. In RPN, every operator encountered evaluates the previous 2 operands, and the result of this operation then becomes the first of the two operands for the subsequent operator. Operands and operators must be spaced by at least one space.
 
+- If the `string` is not valid, or if there is not exactly one argument, `Error` must be printed on the standard output followed by a newline.
+- If the `string` has extra spaces it is still considered valid.
+
 The following operators must be implemented : `+`, `-`, `*`, `/`, and `%`.
 
-If the `string` is not valid, or if there is not exactly one argument, `Error` must be printed on the standard output followed by a newline.
-If the `string` has extra spaces it is still considered valid.
 
 All the given operands must fit in a `i64`.