doc: cp: fix --reflink=when typo in texinfo

This behaviour is correctly documented when doing `cp --help`.
There is no `--reflink=when` option.

* doc/coreutils.texi (cp invocation): Fix document stating
that `--reflink` is equivalent to `--reflink=always`.

doc/coreutils.texi

@@ -9021,7 +9021,7 @@ The @var{when} value can be one of the following:
 @item always
 If the copy-on-write operation is not supported
 then report the failure for each file and exit with a failure status.
-Plain @option{--reflink} is equivalent to @option{--reflink=when}.
+Plain @option{--reflink} is equivalent to @option{--reflink=always}.
 
 @item auto
 If the copy-on-write operation is not supported then fall back