[gcompris] minor: fixed documentation of the memory_add_minus activity.

[Bruno Coudoin <bcoudoin src gnome org>]

commit 7dd5c82398f401b62481e62eba29165c664bd8c6
Author: Bruno Coudoin <bruno coudoin free fr>
Date:   Fri Feb 3 00:21:41 2012 +0100

    minor: fixed documentation of the memory_add_minus activity.

 .../memory_add_minus_tux.xml.in                    |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/memory_add_minus_tux-activity/memory_add_minus_tux.xml.in b/src/memory_add_minus_tux-activity/memory_add_minus_tux.xml.in
index 45f5a0c..426f4de 100644
--- a/src/memory_add_minus_tux-activity/memory_add_minus_tux.xml.in
+++ b/src/memory_add_minus_tux-activity/memory_add_minus_tux.xml.in
@@ -12,7 +12,7 @@
 	 demo="1">
 	 <_title>Addition and subtraction memory game against Tux</_title>
  	 <_description>Turn the cards over to find two numbers which add or subtract the same, until all the cards are gone.</_description>
-	<_prerequisite>Subtraction</_prerequisite>
+	<_prerequisite>Addition and subtraction</_prerequisite>
 	<_goal>Practise addition, subtraction, until all the cards are gone.</_goal>
 	<_manual xml:space="preserve">You can see some cards, but you can't see what's on the other side of them. Each card is hiding an operation, or the answer to it.