[ci skip] readme: fix some syntax errors (#537)

Add missing commas between arguments in some of the code snippets for static controls

Author: yi-jiayu

README.md

@@ -416,15 +416,15 @@ Here's the output for a horizontal layout:
 You can also create a static control that isn't based on a model attribute:
 
 ```erb
-<%= f.static_control :field_name, label: "Custom Static Control" value: "Content Here" %>
+<%= f.static_control :field_name, label: "Custom Static Control", value: "Content Here" %>
 ```
 
 `field_name` may be any name that isn't already used in the form. Note that you may get "unpermitted parameter" messages in your log file with this approach.
 
 You can also create the static control the following way, if you don't need to get the value of the static control as a parameter when the form is submitted:
 
 ```erb
-<%= f.static_control label: "Custom Static Control" value: "Content Here", name: nil %>
+<%= f.static_control label: "Custom Static Control", value: "Content Here", name: nil %>
 ```
 
 (If you neither provide a field name nor `name: nil`, the Rails code that submits the form will give a JavaScript error.)