Fix wrong variable name in assignment in wxRichTextSizePage dtor

The "sm_showMinMaxSizeControls" was assigned twice, while
"sm_enablePositionAndSizeUnits" was not assigned at all, apparently by
mistake.

Thanks to PVS-Studio for finding this issue (V519 The
'sm_showMinMaxSizeControls' variable is assigned values twice successively).

src/richtext/richtextsizepage.cpp

@@ -116,7 +116,7 @@ wxRichTextSizePage::~wxRichTextSizePage()
     sm_showAlignmentControls = true;
     sm_showFloatingAndAlignmentControls = true;
     sm_showMinMaxSizeControls = true;
-    sm_showMinMaxSizeControls = true;
+    sm_enablePositionAndSizeUnits = true;
     sm_enablePositionAndSizeCheckboxes = true;
     sm_showMoveObjectControls = true;