Minor GUI fix needed inside netclass directive dialog [MSW-specific?]

Description

Steps to reproduce

1. Place netclass directive;

KiCad Version

Application: KiCad Schematic Editor (64-bit)

Version: (6.99.0-2103-gdc6c27b686), release build

Libraries:
	wxWidgets 3.1.6
	FreeType 2.11.1
	HarfBuzz 4.2.0
	FontConfig 2.14.0
	libcurl/7.83.1-DEV Schannel zlib/1.2.12

Platform: Windows 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jun  8 2022 15:06:55
	wxWidgets: 3.1.6 (wchar_t,wx containers)
	Boost: 1.79.0
	OCC: 7.6.0
	Curl: 7.83.1-DEV
	ngspice: 36
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON