Minor documentation improvements.
Minor documentation improvements.

- Typo fix.
- Join lines to avoid unwanted vertical whitespace in the output.
- Mark up gvcf like the other options.

vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,brainstorm/bcbio-nextgen,brainstorm/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen