Doc: refer to the "commit-graph file" with dash

Doc: refer to the "commit-graph file" with dash

The file processed by `git commit-graph` is referred to as the
"commit-graph file", also with a dash. We have a few references to the
"commit graph file", though, without the dash. These occur in
git-commit-graph.txt as well as in Doc/technical/commit-graph.txt. Fix
them.

Do not change the references to the "commit graph" (without "... file")
as a data structure.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>