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

Commit 455923e ("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 455923e. 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>

Author: Martin Ågren

Documentation/git-ls-tree.txt

@@ -156,7 +156,7 @@ objectname::
 	The name of the object.
 objectsize[:padded]::
 	The size of the object ("-" if it's a tree).
-	It also supports a padded format of size with "%(size:padded)".
+	It also supports a padded format of size with "%(objectsize:padded)".
 path::
 	The pathname of the object.