osgeo4w: fix designer path

Author: jef-n

ms-windows/osgeo4w/designer.bat.tmpl

@@ -4,4 +4,4 @@ call "%~dp0\qt5_env.bat"
 path %OSGEO4W_ROOT%\apps\@package@\bin;%PATH%
 set QGIS_PREFIX_PATH=%OSGEO4W_ROOT:\=/%/apps/@package@
 set QT_PLUGIN_PATH=%OSGEO4W_ROOT%\apps\@package@\qtplugins;%OSGEO4W_ROOT%\apps\qt5\plugins
-start "Qt Designer with QGIS custom widgets" /B "%OSGEO4W_ROOT%"\bin\designer.exe %*
+start "Qt Designer with QGIS custom widgets" /B designer.exe %*