rebase-interactive: update 'merge' description

rebase-interactive: update 'merge' description

The 'merge' command description for the todo list documentation in an
interactive rebase has multiple lines. The lines other than the first
one start with dots ('.') while the similar multi-line documentation for
'fixup' does not. This description only appears in the comment text of
the todo file during an interactive rebase.

The 'merge' command was documented when interactive rebase was first
ported to C in 145e05ac44b (rebase -i: rewrite append_todo_help() in C,
2018-08-10). These dots might have been carried over from the previous
shell implementation.

The 'fixup' command was documented more recently in 9e3cebd97cb (rebase
-i: add fixup [-C | -c] command, 2021-01-29).

Looking at the output in an editor, my personal opinion is that the dots
are unnecessary and noisy. Remove them now before adding more commands
with multi-line documentation.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>