rewrite_tag_filter: Fix the incorrect pattern

Signed-off-by: Haoyuan Ge <harryge@qq.com>

Author: harryge00

plugins/output/rewrite_tag_filter.md

@@ -26,7 +26,7 @@ prefixed with the value of the key `message`:
   @type rewrite_tag_filter
   <rule>
     key message
-    pattern /^(w+)$/
+    pattern /^\[(\w+)\]/
     tag $1.${tag}
   </rule>
 </match>