DOC: add missing 3.10 in the summary

3.10. Log forwarding was missing in the summary.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 1d077fc..63641a1 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -54,6 +54,7 @@
 3.7.      Programs
 3.8.      HTTP-errors
 3.9.      Rings
+3.10.     Log forwarding
 
 4.    Proxies
 4.1.      Proxy keywords matrix