present2md: fix depth count for blogs with sections
present2md: fix depth count for blogs with sections

Without the bug fix, you get ## followed by #### in the file.

Change-Id: I6af85debf4bc84f9e346732b5641eb3a4e7d7e83
Reviewed-on: https://go-review.googlesource.com/c/tools/+/223602
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools