[gnome-calculator] Use consistent error message

[Robert Roth <robertroth src gnome org>]

commit 79723a81d1fcab7b7b5b13047a3321224adfcf8c
Author: Robert Roth <robert roth off gmail com>
Date:   Wed Jan 4 01:28:34 2017 +0200

    Use consistent error message

 lib/number.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/lib/number.vala b/lib/number.vala
index d0886d4..479a364 100644
--- a/lib/number.vala
+++ b/lib/number.vala
@@ -426,7 +426,7 @@ public class Number : Object
             z.num.@set (num);
             p = n;
         } else {
-            error = _("The zeroth root of a number is undefined.");
+            error = _("The zeroth root of a number is undefined");
             return new Number.integer (0);
         }