fix: use consistent label for Whiteboard editor

"Settings" > "Appearance" > "App bar buttons" > "Whiteboard" > "Whiteboard pen color" actually targets "Whiteboard editor" option in Reviewer.
This commit renames the "Whiteboard pen color" label to "Whiteboard editor" for consistency.

AnkiDroid/src/main/res/values/02-strings.xml

@@ -186,7 +186,6 @@
     <string name="white_board_image_save_failed">Failed to save whiteboard image. %s</string>
     <string name="white_board_image_saved">Whiteboard image saved to %s</string>
 
-    <string name="title_whiteboard_pen_color" maxLength="41">Whiteboard pen color</string>
     <string name="title_whiteboard_editor" maxLength="28">Whiteboard editor</string>
 
     <string name="user_is_a_robot">Detected automated test. If you are a human, contact AnkiDroid support</string>

AnkiDroid/src/main/res/xml/preferences_custom_buttons.xml

@@ -175,7 +175,7 @@ TODO: Add a unit test
             android:entries="@array/custom_button_labels"
             android:entryValues="@array/custom_button_values"
             android:key="@string/custom_button_whiteboard_pen_color_key"
-            android:title="@string/title_whiteboard_pen_color"
+            android:title="@string/title_whiteboard_editor"
             app:useSimpleSummaryProvider="true"/>
         <ListPreference
             android:defaultValue="2"