Fix documentation regarding anchor links

Working with vitepress v1.3.3, I noticed a link in the format `./#heading` does not work / leads to a docs generation error, whereas `#heading` works for me. Hence this PR.

docs/en/guide/markdown.md

@@ -44,7 +44,7 @@ And providing you are in `foo/one.md`:
 ```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 index file -->
+[foo heading](#heading) <!-- anchors user to a heading in the foo index file -->
 [bar - three](../bar/three) <!-- you can omit extension -->
 [bar - three](../bar/three.md) <!-- you can append .md -->
 [bar - four](../bar/four.html) <!-- or you can append .html -->