typos

Co-authored-by: Jonathan Pallant <jonathan.pallant@ferrous-systems.com>

exercise-book/src/calculator.md

@@ -15,9 +15,9 @@ You will learn:
 ## Task
 
 Write a library that parses and evaluates math expressions that use [Reverse-Polish (postfix) notation](https://en.wikipedia.org/wiki/Reverse_Polish_notation).
-For example, a string `"3 1 + 2 /"` in this notation is equivalent to `(3 + 1) / 2` but unlike the later it does not force us to handle operator precedence using parenthesis.
+For example, a string `"3 1 + 2 /"` in this notation is equivalent to `(3 + 1) / 2` but unlike the later it does not force us to handle operator precedence using parentheses.
 
-We will support the 4 basic math operations (`+`, `-`, `*`, `/`) that all will expect two operants, and a `sqr` operation to square a single number.
+We will support the 4 basic math operations (`+`, `-`, `*`, `/`) that all will expect two operands, and a `sqr` operation to square a single number.
 
 Here's a basic grammar of expressions that we can expect:
 
@@ -491,7 +491,7 @@ fn square() {
 
 Some observations:
 
-* Every branch of our big `match` statement eds up producing an expression.
+* Every branch of our big `match` statement ends up producing an expression.
 * Every time we `pop` expressions from the stack we have to box them.
 
 So, here's a plan: