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 <andybons@golang.org>

cmd/present2md/main.go

@@ -172,7 +172,7 @@ func convert(r io.Reader, file string, writeBack bool) error {
 		for _, s := range doc.Sections {
 			fmt.Fprintf(&md, "\n")
 			fmt.Fprintf(&md, "## %s\n", markdownEscape(s.Title, false))
-			printSectionBody(file, 2, &md, s.Elem)
+			printSectionBody(file, 1, &md, s.Elem)
 		}
 	}