Update default_plugins.md

Author: mpearce

docs/default_plugins.md

@@ -32,7 +32,7 @@ require 'mina/default'
  :domain #=> nil
  :port #=> 22
  :deploy_to #=> nil
- :execution_mode #=> :pretty ## Can be [:exec, :pretty, :pritner, :system]
+ :execution_mode #=> :pretty ## Can be [:exec, :pretty, :printer, :system]
 ```
 
 ### Execution mode