Fix typo with full stop (#512)

Author: mathstrains21

concepts/characters/introduction.md

@@ -65,7 +65,7 @@ Strings and Characters can be compared for equality (or lack thereof) using the
 
 ```swift
 let hi = "Hello"
-hi == "Hello."
+hi == "Hello"
 // => true
 
 let period: Character = "."