docs(rpn): correct grammar

subjects/rpn/README.md

@@ -2,14 +2,14 @@
 
 ### Instructions
 
-Write a **program** which takes a `string` that contains an equation written in `Reverse Polish Notation` (RPN) as an argument. The **program** must evaluate the equation, and then:
+Write a **program** which takes a `string` containing an equation written in `Reverse Polish Notation` (RPN). It should evaluate the equation, and print the result on the standard output followed by a newline (`'\n'`).
 
-- 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.
+`Reverse Polish Notation` is a mathematical notation in which every operator follows its operands. Every operator evaluates the previous 2 operands, and the result of this operation becomes the first of the two operands for the subsequent operator. Operands and operators must be spaced by at least one space.
 
 `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.
 
-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`.