List.countby and fixing some small things in seq.countby

[liboz]

Related to #184

[orcabot]

• docs/conceptual/list.countby['t,'key]-function-[fsharp].md - Article contains URLs. Make sure that non-Microsoft URLs are using the aka.ms shortener.
• docs/conceptual/seq.countby['t,'key]-function-[fsharp].md - Article contains URLs. Make sure that non-Microsoft URLs are using the aka.ms shortener.
• docs/conceptual/collections.list-module-[fsharp].md - Article contains URLs. Make sure that non-Microsoft URLs are using the aka.ms shortener.

[orcabot]

Hi! I'm your friendly content validation bot. For this PR to be labeled as ready-to-merge, you'll need at least 2 comments from our designated community approvers containing the magic phrase LGTM (Looks good to me also works, for those of us that are really verbose).

@dend @KevinRansom @cartermp @thedanfernandez @dsyme - please validate this PR.