Update vertex size spinbox step in editing mode

fixes #12943

Set the step of vertex marker size spinbox in Settings menu > Options > Digitizing to 1 instead of 2

Author: DelazJ

src/ui/qgsoptionsbase.ui

@@ -3837,7 +3837,7 @@
                      <number>3</number>
                     </property>
                     <property name="singleStep">
-                     <number>2</number>
+                     <number>1</number>
                     </property>
                    </widget>
                   </item>