Update tuple-structs.md (#1404)

Fix markdown

src/structs/tuple-structs.md

@@ -36,7 +36,7 @@ fn main() {
 
 * Newtypes are a great way to encode additional information about the value in a primitive type, for example:
   * The number is measured in some units: `Newtons` in the example above.
-  * The value passed some validation when it was created, so you no longer have to validate it again at every use: 'PhoneNumber(String)` or `OddNumber(u32)`.
+  * The value passed some validation when it was created, so you no longer have to validate it again at every use: `PhoneNumber(String)` or `OddNumber(u32)`.
 * Demonstrate how to add a `f64` value to a `Newtons` type by accessing the single field in the newtype.
   *  Rust generally doesn’t like inexplicit things, like automatic unwrapping or for instance using booleans as integers.
   *  Operator overloading is discussed on Day 3 (generics).