Fix declaration syntax of $(uppercase) template function

Signed-off-by: Hofi <hofione@gmail.com>

Author: HofiOne

doc/_admin-guide/110_Template_and_rewrite/000_Customize_message_format/007_Template_functions_of_syslog-ng.md

@@ -1162,7 +1162,7 @@ $(tags-head <name-of-first-tag> <name-of-second-tag> ... )
 template("$(tags-head foo bar baz)")
 ```
 
-## uppercase
+## $(uppercase)
 
 | *Syntax:* | $(uppercase "\<macro\>") |