Update index.md

Co-authored-by: David Stansby <dstansby@gmail.com>

Author: jo-mueller

index.md

@@ -452,7 +452,7 @@ They:
 - MUST contain the field `output`, which is an object with fields `name` and `path`.
   The `output` field MAY be omitted if the transformation is part of a wrapper transform
   (i.e., [`sequence`](#sequence-md), [`bijection`](#bijection-md), [`byDimension`](#bydimension-md), see details).
-- MUST contain the field `input` (string), which is an object with fields `name` and `path`.
+- MUST contain the field `input`, which is an object with fields `name` and `path`.
   The `input` field MAY be omitted if the transformation is part of a wrapper transform
   (i.e., [`sequence`](#sequence-md), [`bijection`](#bijection-md), [`byDimension`](#bydimension-md), see details).
 - MAY contain the field `name` (string).