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

[Bruno Coudoin <bcoudoin src gnome org>]

commit 5dcfa9cb0198e20fa881360876a9515e614ccef7
Author: Bruno Coudoin <bruno coudoin free fr>
Date:   Thu Feb 2 23:48:22 2012 +0100

    minor: fixed documentation of the memory_add_minus activity.

 .../memory_add_minus.xml.in                        |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/memory_add_minus-activity/memory_add_minus.xml.in b/src/memory_add_minus-activity/memory_add_minus.xml.in
index f2ffff2..fc931c6 100644
--- a/src/memory_add_minus-activity/memory_add_minus.xml.in
+++ b/src/memory_add_minus-activity/memory_add_minus.xml.in
@@ -12,7 +12,7 @@
 	 demo="1">
 	 <_title>Addition and subtraction memory game</_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.