doc : add a dot in structs3.rs file

exercises/07_structs/structs3.rs

@@ -1,7 +1,7 @@
 // structs3.rs
 //
 // 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.
 // Make the code compile and the tests pass!
 //
 // Execute `rustlings hint structs3` or use the `hint` watch subcommand for a