Fix spacing for telegraf_plugins_extra example in readme.md (#143)

The spacing for the telegraf_plugins_extra section (lines 273-276) in readme.md was too indented, corrected spacing to hopefully be valid YML.

README.md

@@ -271,9 +271,9 @@ The 2nd parameter `telegraf_plugins_extra` can be used to add plugins specific t
 
 	cat group_vars/mysql_database
 	telegraf_plugins_extra:
-		mysql:
-		  config:
-		  	- servers = ["root:{{ mysql_root_password }}@tcp(localhost:3306)/"]
+	  mysql:
+	    config:
+	      - servers = ["root:{{ mysql_root_password }}@tcp(localhost:3306)/"]
 
 There is an option to delete extra-plugin files in /etc/telegraf/telegraf.d if they weren't generated by this playbook with `telegraf_plugins_extra_exclusive` variable.