It was probably just an oversight: the `--recurse-submodules` option
puts the term "submodules" in the plural form, not the singular one.
To avoid future problems in case that another option is introduced that
starts with the prefix `--recurse-submodule`, let's just fix this.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>