Update website/pages/docs/secrets/transform/index.mdx

Co-authored-by: Calvin Leung Huang <[email protected]>

Author: sgmiller

website/pages/docs/secrets/transform/index.mdx

@@ -250,7 +250,7 @@ The following builtin alphabets are available for use in the secret engine:
 - builtin/alphanumericupper
 - builtin/alphanumeric
 
-Custom alphabets may contain between 2 and 65536 characters.
+Custom alphabets must contain between 2 and 65536 unique characters.
 
 ## Learn