Use rooted URL for the image in SECURITY.md

Turns out the security policy document is accessible on GitHub from at
least two paths, and the relative image URL only worked in one and not
the other. Using a rooted URL fixes that problem.

Author: mhucka

.github/SECURITY.md

@@ -13,7 +13,7 @@ repository page.
 
 <div align="center">
 <img width="75%" alt="Location of the report button on the repository page"
-    src="./report-vulnerability-button.png">
+    src="/.github/report-vulnerability-button.png">
 </div>
 
 Please report security issues in third-party modules to the person or team