Docs: Fix builds.md line-length + minor typo issues

This PR proposes to fix some already existing (i.e., older)
line-length & line-alignment issues and the ones recently
included/merged with the PR #7406
(https://github.com/docker/docker/pull/7406).

It also proposes to fix a couple minor typography mistakes
(i.e., spelling of GitHub and Service Hooks (headers))

This PR does *not* propose any content changes.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
fix for issue 7281
add missing comma per O.S. Tezer's commment

Signed-off-by: Doug Davis <dug@us.ibm.com>
Update gentoolinux.md
fix command output examples in cli documentation

f87a97f accidentally added a few too many `$` line prefixes

Docker-DCO-1.1-Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com> (github: phemmer)
Merge pull request #7514 from phemmer/cli_cmd_prefix_fix

fix command output examples in cli documentation
Merge pull request #7501 from wyc/patch-1

Update gentoolinux.md
Merge pull request #7498 from ostezer/fix-builds-errors

Docs: Fix builds.md line-length + minor typo issues
Merge pull request #7489 from duglin/7281

fix for issue 7281