Fix typo in structs3.rs

Author: yerke

exercises/07_structs/structs3.rs

@@ -1,5 +1,5 @@
 // Structs contain data, but can also have logic. In this exercise we have
-// defined the `Package` struct and we want to test some logic attached to it.
+// defined the `Package` struct, and we want to test some logic attached to it.
 
 #[derive(Debug)]
 struct Package {