DOC: configuration: http-check send was missing from matrix

The new directive and its doc were added by commit 8acb1284b ("MINOR:
checks: Add a way to send custom headers and payload during http chekcs")
but the index was not updated.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index c75bb55..d72dca6 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2788,6 +2788,7 @@
 http-check connect                        X          -         X         X
 http-check disable-on-404                 X          -         X         X
 http-check expect                         X          -         X         X
+http-check send                           X          -         X         X
 http-check send-state                     X          -         X         X
 http-check set-var                        X          -         X         X
 http-check unset-var                      X          -         X         X