minor copyedit on theme import error

minor copyedit on theme import error

diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 996fd40..06f0231 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -81,7 +81,7 @@ en:
       unrecognized_extension: "Unrecognized file extension: %{extension}"
     import_error:
       generic: An error occured while importing that theme
-      about_json: "Import Error: about.json does not exist, or is invalid"
+      about_json: "Import Error: about.json does not exist, or is invalid. Are you sure this is a Discourse Theme?"
       about_json_values: "about.json contains invalid values: %{errors}"
       modifier_values: "about.json modifiers contain invalid values: %{errors}"
       git: "Error cloning git repository, access is denied or repository is not found"

GitHub sha: 2660c2e2