[gimp-help] Updated option name

[Marco Ciampa <marcoc src gnome org>]

commit a10e0c5f2e1c12d12482beb0d575f3dc28bc1677
Author: Marco Ciampa <ciampix posteo net>
Date:   Fri Apr 3 14:27:12 2020 +0200

    Updated option name

 src/menus/colors/colortoalpha.xml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/menus/colors/colortoalpha.xml b/src/menus/colors/colortoalpha.xml
index 7a9d3f45e..e954616f7 100644
--- a/src/menus/colors/colortoalpha.xml
+++ b/src/menus/colors/colortoalpha.xml
@@ -70,7 +70,7 @@
           </para>
           <para>
             <quote>Color to alpha</quote> comes with a GEGL option dialog under 
-            Toolbox. When the <guilabel>Sample merged</guilabel> option is 
+            Toolbox. When the <guilabel>Sample average</guilabel> option is 
             unchecked, color picker picks only one pixel color. When the option 
             is checked, mouse pointer goes with a square limiting a pixel sample 
             that determines a merged sample <quote>color</quote>. You can fix