git-tag.txt: Add a missing hyphen to `-s`

Signed-off-by: Wieland Hoffmann <themineo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/mbox-from-line' into maint

Some MUAs mangled a line in a message that begins with "From " to
">From " when writing to a mailbox file and feeding such an input to
"git am" used to lose such a line.

* jk/mbox-from-line:
  mailinfo: work around -Wstring-plus-int warning
  mailinfo: make ">From" in-body header check more robust
Merge branch 'maint-1.8.5' into maint-1.9

* maint-1.8.5:
  git-tag.txt: Add a missing hyphen to `-s`
Merge branch 'maint-1.9' into maint-2.0

* maint-1.9:
  git-tag.txt: Add a missing hyphen to `-s`
Merge branch 'maint-2.0' into maint

* maint-2.0:
  git-tag.txt: Add a missing hyphen to `-s`
Sync with maint

* maint:
  git-tag.txt: Add a missing hyphen to `-s`