git-column.txt: fix section header

We have too few dashes under "Examples", which causes Asciidoctor to not pick it up as a section header. Instead, it thinks we are starting a code listing block, which ends up containing the remainder of the document. The result is quite ugly. Make sure we have as many dashes as we have characters in "Examples". Asciidoc renders identically before and after this patch, both man-page and html. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>