add text param to docs and fix inheritance error

Author: pascalwengerter

app/concepts/matestack/ui/core/sub/sub.rb

@@ -1,4 +1,4 @@
-module Matestack::Ui::Core::Span
+module Matestack::Ui::Core::Sub
   class Sub < Matestack::Ui::Core::Component::Static
 
   end

docs/components/sub.md

@@ -14,6 +14,9 @@ Expects a string with all ids the sub should have.
 #### # class (optional)
 Expects a string with all classes the sub should have.
 
+#### # text (optional)
+Expects a string with the text that should go into the `<sub>` tag.
+
 ## Example 1: Yield a given block
 
 ```ruby