Fix line numbers

Fixes #69.

variables/variables1.rs

@@ -38,5 +38,5 @@ fn main() {
 
 
 
-// Hint: The declaration on line 4 is missing a keyword that is needed in Rust
+// Hint: The declaration on line 5 is missing a keyword that is needed in Rust
 // to create a new variable binding.

variables/variables2.rs

@@ -40,7 +40,7 @@ fn main() {
 
 // The compiler message is saying that Rust cannot infer the type that the
 // variable binding `x` has with what is given here.
-// What happens if you annotate line 4 with a type annotation?
+// What happens if you annotate line 5 with a type annotation?
 // What if you give x a value?
 // What if you do both?
 // What type should x be, anyway?

variables/variables4.rs

@@ -39,7 +39,7 @@ fn main() {
 
 
 // Oops! In this exercise, we have a variable binding that we've created on
-// line 4, and we're trying to use it on line 5, but we haven't given it a
+// line 5, and we're trying to use it on line 6, but we haven't given it a
 // value. We can't print out something that isn't there; try giving x a value!
 // This is an error that can cause bugs that's very easy to make in any
 // programming language -- thankfully the Rust compiler has caught this for us!