CLEANUP: log: fix comment of parse_logformat_string()

"fmt" is passed to parse_logformat_string, adjust comment
accordingly
diff --git a/src/log.c b/src/log.c
index c2153b8..5204de2 100644
--- a/src/log.c
+++ b/src/log.c
@@ -551,7 +551,7 @@
  * You can set arguments using { } : %{many arguments}varname.
  * The curproxy->conf.args.ctx must be set by the caller.
  *
- *  str: the string to parse
+ *  fmt: the string to parse
  *  curproxy: the proxy affected
  *  list_format: the destination list
  *  options: LOG_OPT_* to force on every node