binutils: Fix broken numbering in strip/objcopy --only-keep-debug doc

The documentation for --only-keep-debug displayed the procedure steps as
"1. 1. 1. 1." in the man pages, despite using the @enumerate command.

This patch moves the item text to the line following the @item tag and
adds whitespace between items. This forces the man page generator to
correctly distinguish the items and increment the list counter.

binutils/
        * doc/binutils.texi (strip, objcopy): Reformat @enumerate list
        to fix man page numbering.