Apply (pronto-)rubocop suggestions for PR #197feat_improve_menu

```
$ pronto run
lib/highline/menu.rb:371 I: Prefer `to_s` over string interpolation.
lib/highline/menu/item.rb:5 I: Missing top-level class
  documentation comment.
lib/highline/menu/item.rb:11 I: Line is too long. [99/80]
lib/highline/menu/item.rb:13 I: Line is too long. [91/80]
lib/highline/menu/item.rb:28 I: Final newline missing.
```

2 files changed, 7 insertions, 5 deletions

diff --git a/lib/highline/menu.rb b/lib/highline/menu.rb
index a942d99..8dc9247 100644
--- a/lib/highline/menu.rb
+++ b/lib/highline/menu.rb
@@ -368,7 +368,7 @@ class HighLine
     def map_items_by_index
       if @index == :letter
         l_index = "`"
-        all_items.map { "#{l_index.succ!}" }
+        all_items.map { l_index.succ!.dup }
       else
         (1..all_items.size).map(&:to_s)
       end
diff --git a/lib/highline/menu/item.rb b/lib/highline/menu/item.rb
index 3e670a3..76e1e56 100644
--- a/lib/highline/menu/item.rb
+++ b/lib/highline/menu/item.rb
@@ -1,16 +1,18 @@
 class HighLine
   class Menu < Question
     # Represents an Item of a HighLine::Menu.
-
+    #
     class Item
       attr_reader :name, :text, :help, :action
 
       #
       # @param name [String] The name that is matched against the user input
       # @param attributes [Hash] options Hash to tailor menu item to your needs
-      # @option attributes text: [String] The text that displays for that choice (defaults to name)
+      # @option attributes text: [String] The text that displays for that
+      #   choice (defaults to name)
       # @option attributes help: [String] help/hint string to be displayed.
-      # @option attributes action: [Block] a block that gets called when choice is selected
+      # @option attributes action: [Block] a block that gets called when choice
+      #   is selected
       #
       def initialize(name, attributes)
         @name = name
@@ -25,4 +27,4 @@ class HighLine
       end
     end
   end
-end
\ No newline at end of file
+end