[DOC] renamed 'haproxy' instead of 'ha-proxy' or 'h a p r o x y'

File indexation systems (including search engines) did not read
'haproxy' when needed, which made it more difficult to find the
doc on the net.
diff --git a/doc/architecture.txt b/doc/architecture.txt
index 361ecf1..8b04f99 100644
--- a/doc/architecture.txt
+++ b/doc/architecture.txt
@@ -1,5 +1,5 @@
                            -------------------
-                             H A - P r o x y
+                                 HAProxy
                            Architecture  Guide
                            -------------------
                              version 1.1.34