[gimp-help] Add missing help id gimp-edit-undo-clear to Undo History Dialog

[Jacob Boerema <jboerema src gnome org>]

commit 60c76da4fddfcd0d21f5dffd5ae4a380275acec4
Author: Jacob Boerema <jgboerema gmail com>
Date:   Sun Oct 16 15:58:38 2022 -0400

    Add missing help id  gimp-edit-undo-clear to Undo History Dialog
    
    (cherry picked from commit 29d31023868316431c7a576b5de7d16291c1944a)

 src/dialogs/undo-history-dialog.xml | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/dialogs/undo-history-dialog.xml b/src/dialogs/undo-history-dialog.xml
index 8519702d6..b717db816 100644
--- a/src/dialogs/undo-history-dialog.xml
+++ b/src/dialogs/undo-history-dialog.xml
@@ -138,6 +138,7 @@
           <phrase condition="tag-is-required">Clear Undo History</phrase>
         </term>
         <listitem>
+          <anchor id="gimp-edit-undo-clear"/>
           <para>
             This button removes all contents from the undo history except the
             current state. If you press it, you are asked to confirm that you