[shotwell] Replace deprecated string function

[Jens Georg <jensgeorg src gnome org>]

commit a8fe9eafef635f75c20b3ecc78cb712491a36ba8
Author: Jens Georg <mail jensge org>
Date:   Fri Dec 20 20:59:00 2019 +0100

    Replace deprecated string function

 src/dialogs/AdjustDateTimeDialog.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/dialogs/AdjustDateTimeDialog.vala b/src/dialogs/AdjustDateTimeDialog.vala
index 3834f99f..9ca41d60 100644
--- a/src/dialogs/AdjustDateTimeDialog.vala
+++ b/src/dialogs/AdjustDateTimeDialog.vala
@@ -123,7 +123,7 @@ public class AdjustDateTimeDialog : Gtk.Dialog {
         for (int i = 0; i < 12; i++){
             var dt = new DateTime.from_unix_utc(i * 2764800);
             var month_string = dt.format("%OB");
-            if (month_string.str("%OB") != null) {
+            if (month_string.index_of("%OB") != -1) {
                 month_string = dt.format("%B");
             }