Fixed type hint failure for Python <= 3.8
Fixed type hint failure for Python <= 3.8

Python <= 3.8 interprets "list" (lowercase) as a type and throws an error when trying to index into it as a type-hint.  Converted to uppercase "List".

PiperOrigin-RevId: 471238736
Change-Id: Ic5d7a529c8faa6dc17f2ba148303d8f796065f4e

deepmind/acme,deepmind/acme