docs: Fix dead link to visual route editor (prometheus#2298) (prometheus#2300)

Relative links are rendered relatively to the source repository, so this
link points to GitHub at the moment.

This commit changes the link to an absolute URL, bringing readers to the
Routing tree editor.

Signed-off-by: Manuel Hutter <[email protected]>

Co-authored-by: Manuel Hutter <[email protected]>

Author: Julien Pivotto

docs/configuration.md

@@ -12,8 +12,8 @@ While the command-line flags configure immutable system parameters, the
 configuration file defines inhibition rules, notification routing and
 notification receivers.
 
-The [visual editor](/webtools/alerting/routing-tree-editor) can assist in
-building routing trees.
+The [visual editor](https://www.prometheus.io/webtools/alerting/routing-tree-editor)
+can assist in building routing trees.
 
 To view all available command-line flags, run `alertmanager -h`.