Add dot suffix to headings in guides

This makes the numbering in the guide more clear and separate from the
actual headings.

Example:

    ## Introduction

    ### Welcome to Rails

Before:

    1 Introduction

    1.1 Welcome to Rails

After:

    1. Introduction

    1.1. Welcome to Rails

Author: sikachu

guides/rails_guides/markdown.rb

@@ -185,15 +185,15 @@ def node_index(hierarchy)
         case hierarchy.size
         when 1
           @index_counter[2] = @index_counter[3] = @index_counter[4] = 0
-          "#{@index_counter[1] += 1}"
+          "#{@index_counter[1] += 1}."
         when 2
           @index_counter[3] = @index_counter[4] = 0
-          "#{@index_counter[1]}.#{@index_counter[2] += 1}"
+          "#{@index_counter[1]}.#{@index_counter[2] += 1}."
         when 3
           @index_counter[4] = 0
-          "#{@index_counter[1]}.#{@index_counter[2]}.#{@index_counter[3] += 1}"
+          "#{@index_counter[1]}.#{@index_counter[2]}.#{@index_counter[3] += 1}."
         when 4
-          "#{@index_counter[1]}.#{@index_counter[2]}.#{@index_counter[3]}.#{@index_counter[4] += 1}"
+          "#{@index_counter[1]}.#{@index_counter[2]}.#{@index_counter[3]}.#{@index_counter[4] += 1}."
         end
       end