Update spec/Section 2 -- Language.md

Co-authored-by: Benjie <benjie@jemjie.com>

Author: MarkoMin

spec/Section 2 -- Language.md

@@ -419,8 +419,7 @@ discrete piece of information available to request within a selection set.
 Some fields describe complex data or relationships to other data. In order to
 further explore this data, a field may itself contain a selection set, allowing
 for deeply nested requests. All GraphQL operations must specify their selections
-down to fields which return scalar or enum values to ensure an unambiguously
-shaped response.
+down to _leaf fields_ to ensure an unambiguously shaped response.
 
 For example, this operation selects fields of complex data and relationships
 down to scalar values.