Thank you for reaching out to us Mahtab.
To change this text we really suggest to use the Loco Translate plugin:
https://fr.wordpress.org/plugins/loco-translate/
Once installed, go to Dashboard > Loco translate > Plugins > rtMedia for WordPress, BuddyPress and bbPress. The first time, perhaps you’ll have to create a new language. In your case, select the language of your website. Then, search the word you want to change.
If you don’t find this string, I suggest you click on “Synchronize”. It will automatically add the new strings.
Best,