l10n: Grammar: url -> URL

Signed-off-by: rakekniven <mark.ziegler@rakekniven.de>

src/main/res/values/strings.xml

@@ -879,7 +879,7 @@
     <string name="retrieving_file">Retrieving file…</string>
     <string name="associated_account_not_found">Associated account not found!</string>
     <string name="error_retrieving_file">Error retrieving file</string>
-    <string name="could_not_retrieve_url">Could not retrieve url</string>
+    <string name="could_not_retrieve_url">Could not retrieve URL</string>
     <string name="failed_to_download">Failed to pass file to download manager</string>
     <string name="failed_to_print">Failed to print file</string>
     <string name="autoupload_disable_power_save_check">Disable power save check</string>