Fixes example snippets in StringUTF8View.swift

Author: valeriyvan

stdlib/public/core/StringUTF8View.swift

@@ -78,15 +78,15 @@ extension String {
   ///
   ///     print(strncmp(s1, s2, 14))
   ///     // Prints "0"
-  ///     print(String(s1.utf8.prefix(14)))
+  ///     print(String(s1.utf8.prefix(14))!)
   ///     // Prints "They call me '"
   ///
   /// Extending the compared character count to 15 includes the differing
   /// characters, so a nonzero result is returned.
   ///
   ///     print(strncmp(s1, s2, 15))
   ///     // Prints "-17"
-  ///     print(String(s1.utf8.prefix(15)))
+  ///     print(String(s1.utf8.prefix(15))!)
   ///     // Prints "They call me 'B"
   @frozen
   public struct UTF8View {