add credits for image used in screenshot (#7040)

hey/readme.txt

@@ -36,6 +36,12 @@ but WITHOUT ANY WARRANTY; without even the implied warranty of
 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 GNU General Public License for more details.
 
+This theme bundles the following third-party resources:
+
 Piazzolla Font
 Licensed under SIL Open Font License, 1.1 (http://scripts.sil.org/OFL)
 Source: https://fonts.google.com/specimen/Piazzolla
+
+Confident Asian woman against green background (screenshot.png)
+License: https://www.pexels.com/license/
+Source: https://www.pexels.com/photo/confident-asian-woman-against-green-background-4906334/

hey/style.css

@@ -7,7 +7,7 @@ Description: Hey is a simple personal blog theme.
 Requires at least: 6.1
 Tested up to: 6.1
 Requires PHP: 5.7
-Version: 1.0.2
+Version: 1.0.3
 License: GNU General Public License v2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html
 Template: