OF_node_from_xref.9: Fix function name typo + spdx

MFC after:		3 days
Reported by:		Setesh Strong on Community Discord
Fixes:			ac32b99c2b (Add documentation for OpenFirmware)
Reviewed by:		carlavilla, kevans
Approved by:		carlavilla (mentor)
Differential Revision:	https://reviews.freebsd.org/D49653

share/man/man9/OF_node_from_xref.9

@@ -1,4 +1,6 @@
 .\"
+.\" SPDX-License-Identifier: BSD-2-Clause
+.\"
 .\" Copyright (c) 2018 Oleksandr Tymoshenko <[email protected]>
 .\"
 .\" All rights reserved.
@@ -70,9 +72,9 @@ If one cannot be found or the OpenFirmware implementation
 does not support effective phandles, the function returns
 the input value.
 .Pp
-.Fn OF_xref_from_xref
+.Fn OF_xref_from_node
 returns the effective phandle for the kernel phandle
-.Fa xref .
+.Fa node .
 If one cannot be found or the OpenFirmware implementation
 does not support effective phandles, the function returns
 the input value.