Add a new image in the markdown help page

Previously we were linking directly from the public/
directory. While this worked for the GitLab help page,
it was not working for doc.gitlab.com. Adding an image
in a relative directory and linking from there serves
both ends.

[ci skip]

doc/markdown/markdown.md

@@ -424,24 +424,24 @@ will point the link to `wikis/style` when the link is inside of a wiki markdown
     Here's our logo (hover to see the title text):
 
     Inline-style:
-    ![alt text](assets/logo.svg)
+    ![alt text](img/logo.png)
 
     Reference-style:
     ![alt text1][logo]
 
-    [logo]: assets/logo.svg
+    [logo]: img/logo.png
 
 Here's our logo:
 
 Inline-style:
 
-![alt text](/assets/logo.svg)
+![alt text](img/logo.png)
 
 Reference-style:
 
 ![alt text][logo]
 
-[logo]: /assets/logo.svg
+[logo]: img/logo.png
 
 ## Blockquotes