git-ls-tree.txt: fix the name of "%(objectsize:padded)"

git-ls-tree.txt: fix the name of "%(objectsize:padded)"

Commit 455923e0a1 ("ls-tree: introduce "--format" option", 2022-03-23)
introduced `--format` and the various placeholders it can take, such as
%(objectname) and %(objectsize).

At some point when that patch was being developed, those placeholders
had shorter names, e.g., %(name) and %(size), which can be seen in the
commit message of 455923e0a1. One instance of "%(size:padded)" also
managed to enter the documentation in the final version of the patch.
Correct it to "%(objectsize:padded)".

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>