Use SPDX identifier in license field of META.info

Use the standardized identifier for the license field.
For more details see https://design.perl6.org/S22.html#license

Author: samcv

META.info

@@ -3,6 +3,7 @@
     "name"        : "p6doc",
     "version"     : "*",
     "description" : "Perl 6 documentation (tools and docs)",
+    "license"     : "Artistic-2.0",
     "depends"     : [ "URI", "File::Temp", "JSON::Fast", "Pod::To::BigPage", "Pod::To::HTML",
                       "OO::Monitors", "IO::String" ],
     "build-depends" : [ "File::Find" ],