docs: fix invalid code block language (#12734)

This is causing an error while importing docs into our website

Author: AlessioGr

docs/plugins/form-builder.mdx

@@ -474,7 +474,7 @@ formBuilderPlugin({
 
 Plugin fields can cause GraphQL type name collisions with your own blocks or collections. This results in errors like:
 
-```txt
+```plaintext
 Error: Schema must contain uniquely named types but contains multiple types named "Country"
 ```