Fix description of enums supported by contract types (#703)

docs/soroban-internals/types/custom-types.mdx

@@ -97,8 +97,7 @@ element vector, where the first element is the name of the enum variant as a
 string up to 32 characters in length, and the value is the value if the variant
 has one.
 
-Only unit variants and single value variants, like `A` and `B` below, are
-supported.
+Only unit variants and tuple variants, like `A` and `B` below, are supported.
 
 ```rust
 #[contracttype]