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

cmd/bio-bam-sort/cmd/main.go

@@ -100,9 +100,9 @@ sorting and merging.
 
    The command reads a sequence of bam or sam records from input, sorts them,
    and produces file <output.sortshard> (note: this is NOT a BAM formatted file,
-   it is a shard file). If <input> is '-', records are read from stdin.  With
-   -sam flag, the records are assumed to be in SAM format. Else, it is assumed
-   to be in BAM format.
+   it is a shard file). If <input> is '-', records are read from stdin. If
+   -sam=false, records are assumed to be in the BAM format. Else, records are
+   assumed to be in SAM format.
 
 2. bio-bam-sort -bam <foo.bam> <input.sortshard...>