docs: modify README.md to index.md

docs/guide/markdown.md

@@ -28,9 +28,9 @@ For example, given the following directory structure:
 And providing you are in `foo/one.md`:
 
 ```md
-[Home](/) <!-- sends the user to the root README.md -->
+[Home](/) <!-- sends the user to the root index.md -->
 [foo](/foo/) <!-- sends the user to index.html of directory foo -->
-[foo heading](./#heading) <!-- anchors user to a heading in the foo README file -->
+[foo heading](./#heading) <!-- anchors user to a heading in the foo index file -->
 [bar - three](../bar/three) <!-- you can omit extention -->
 [bar - three](../bar/three.md) <!-- you can append .md -->
 [bar - four](../bar/four.html) <!-- or you can append .html -->