Convert [] in heading to -squarebrackets in id

This essentially fixes an id collision when the guide has both method
name and method name with square brackets in the heading.

Example:

    # Working with validation errors

    ## `errors`

    ## `errors[]`

Before:

    #working-with-validation-errors

    #working-with-validation-errors-errors

    #working-with-validation-errors-errors <= duplicate

After:

    #working-with-validation-errors

    #working-with-validation-errors-errors

    #working-with-validation-errors-errors-squarebrackets

Author: sikachu

guides/rails_guides/markdown.rb

@@ -55,6 +55,7 @@ def dom_id_text(text)
 
         text.downcase.gsub(/\?/, "-questionmark")
                      .gsub(/!/, "-bang")
+                     .gsub(/\[\]/, "-squarebrackets")
                      .gsub(/[#{escaped_chars}]+/, " ").strip
                      .gsub(/\s+/, "-")
       end