[gnome-calculator] Mention tau in help (refs #46)

[Robert Roth <robertroth src gnome org>]

commit 5ca6af51906586eb960142bedb9ee2a5bd4277b9
Author: Robert Roth <robert roth off gmail com>
Date:   Thu Jun 4 17:47:44 2020 +0300

    Mention tau in help (refs #46)

 help/C/variables.page | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/help/C/variables.page b/help/C/variables.page
index fa88608f..a4492032 100644
--- a/help/C/variables.page
+++ b/help/C/variables.page
@@ -49,6 +49,10 @@
     <td><p><link xref="trigonometry">Pi</link></p></td>
     </tr>
     <tr>
+    <td><p>τ</p></td>
+    <td><p><link xref="trigonometry">Tau</link></p></td>
+    </tr>
+    <tr>
     <td><p>rand</p></td>
     <td><p>Random value in the range [0,1] (changes on each read)</p></td>
     </tr>