DOC: log: more than 2 log servers are allowed

Since commit 0f99e3497, loggers are not limited to 2 instances anymore.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index d6f8b8d..c2e4435 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -818,7 +818,7 @@
   See also "gid" and "user".
 
 log <address> [len <length>] [format <format>] <facility> [max level [min level]]
-  Adds a global syslog server. Up to two global servers can be defined. They
+  Adds a global syslog server. Several global servers can be defined. They
   will receive logs for starts and exits, as well as all logs from proxies
   configured with "log global".