objcopy: fix typo in --heap and --stack parser

The help says that <reserve> and <commit> should be separated by a ","
but the implementation is checking for ".". Having two numbers being
separated by a "." could be confusing, thus adjust the implementation to
match the help syntax.

binutils/ChangeLog:

	* objcopy.c (copy_main): Set separator to "," between <reserve>
	and <commit> for --heap and --stack.
	* doc/binutils.texi: Add <commit> for --heap and --stack.