[gnome-calculator] Use a Unicode apostrophe in a new translatable string

[Piotr Drąg <piotrdrag src gnome org>]

commit 3898772da397f4c3d15d161e5aa56a5b2f9a0042
Author: Piotr Drąg <piotrdrag gmail com>
Date:   Fri Jan 26 18:34:45 2018 +0100

    Use a Unicode apostrophe in a new translatable string
    
    See https://developer.gnome.org/hig/stable/typography.html

 lib/serializer.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/lib/serializer.vala b/lib/serializer.vala
index e2bdbcc..defb9c2 100644
--- a/lib/serializer.vala
+++ b/lib/serializer.vala
@@ -329,7 +329,7 @@ public class Serializer : Object
             else
             {
                 string.prepend_c ('?');
-                error = _("Overflow: the result couldn't be calculated");
+                error = _("Overflow: the result couldn’t be calculated");
                 string.assign ("0");
                 break;
             }