Fix string: Adding group members

This changes the string "Add member" to "Add members" as the
corresponding list is multi-selectable.

Closes #5697
// FREEBIE

res/values/strings.xml

@@ -914,7 +914,7 @@
 
     <!-- recipients_panel -->
     <string name="recipients_panel__to"><small>Enter a name or number</small></string>
-    <string name="recipients_panel__add_member">Add member</string>
+    <string name="recipients_panel__add_members">Add members</string>
 
     <!-- verify_display_fragment -->
     <string name="verify_display_fragment__scan_the_code_on_your_contact_s_phone_or_ask_them_to_scan_your_code_to_verify_that_your_messages_are_end_to_end_encrypted_you_can_alternately_compare_the_number_above">If you wish to verify the security of your end-to-end encryption with %s, compare the numbers above with the numbers on their device. Alternately, you can scan the code on their phone, or ask them to scan your code.</string>

src/org/thoughtcrime/securesms/GroupCreateActivity.java

@@ -198,7 +198,7 @@ public class GroupCreateActivity extends PassphraseRequiredActionBarActivity
     SelectedRecipientsAdapter adapter = new SelectedRecipientsAdapter(this);
     adapter.setOnRecipientDeletedListener(this);
     lv.setAdapter(adapter);
-    recipientsEditor.setHint(R.string.recipients_panel__add_member);
+    recipientsEditor.setHint(R.string.recipients_panel__add_members);
     recipientsPanel.setPanelChangeListener(this);
     findViewById(R.id.contacts_button).setOnClickListener(new AddRecipientButtonListener());
     avatar.setImageDrawable(ContactPhotoFactory.getDefaultGroupPhoto()