[DOC] fix another typo in the doc

Config check is -c, not -d. Reported by David Dunaway.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 2dab673..18c4047 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -417,7 +417,7 @@
 
 Assuming haproxy is in $PATH, test these configurations in a shell with:
 
-    $ sudo haproxy -f configuration.conf -d
+    $ sudo haproxy -f configuration.conf -c
 
 
 3. Global parameters