DOC: fix index to reference bind and server options

Last commit forgot to update the index.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 149c0c8..a270d69 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -53,7 +53,9 @@
 4.1.      Proxy keywords matrix
 4.2.      Alphabetically sorted keywords reference
 
-5.    Server and default-server options
+5.    Bind and Server options
+5.1.      Bind options
+5.2.      Server and default-server options
 
 6.    HTTP header manipulation