docs/manual: use SPDX short identifier for license

Signed-off-by: Rahul Bedarkar <rahulbedarkar89@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 08bbbe73af0d60ffac4f4476d0583764b81fdfac)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-cargo.txt

@@ -41,7 +41,7 @@ package. Let's start with an example:
 07: FOO_VERSION = 1.0
 08: FOO_SOURCE = foo-$(FOO_VERSION).tar.gz
 09: FOO_SITE = http://www.foosoftware.org/download
-10: FOO_LICENSE = GPLv3+
+10: FOO_LICENSE = GPL-3.0+
 11: FOO_LICENSE_FILES = COPYING
 12:
 13: FOO_DEPENDENCIES = host-cargo

docs/manual/adding-packages-meson.txt

@@ -24,7 +24,7 @@ package. Let's start with an example:
 07: FOO_VERSION = 1.0
 08: FOO_SOURCE = foo-$(FOO_VERSION).tar.gz
 09: FOO_SITE = http://www.foosoftware.org/download
-10: FOO_LICENSE = GPLv3+
+10: FOO_LICENSE = GPL-3.0+
 11: FOO_LICENSE_FILES = COPYING
 12: FOO_INSTALL_STAGING = YES
 13: