cmd/bio-bam-sort: fix bio-bam-sort -sam documentation

Summary:
Fix documentation for `bio-bam-sort -sam ...`
to state that `-sam=true` by default and if
a user wants to sort BAM records, they must
explicitly state `-sam=false`.

For more context on this change, see D47837.

Test Plan:
Proofread documentation changes.
There are no code changes to test.

Reviewers: cchang

Reviewed By: cchang

Subscribers: sbanville, ayip

Tags: #methylation_pipeline

Maniphest Tasks: T39542

Differential Revision: https://phabricator.grailbio.com/D47856

fbshipit-source-id: 67c1f09