docs: fix broken example in body-transformer (#11409)

diff --git a/docs/en/latest/plugins/body-transformer.md b/docs/en/latest/plugins/body-transformer.md
index 15df4db..dc1b786 100644
--- a/docs/en/latest/plugins/body-transformer.md
+++ b/docs/en/latest/plugins/body-transformer.md
@@ -138,6 +138,7 @@
     "plugins": {
         "body-transformer": {
             "request": {
+                "template_is_base64": true,
                 "template": "'"$(base64 -w0 /path/to/my_template_file)"'"
             }
         }