prboom: Use lowercase for kconfig option text

Like elsewhere in Buildroot.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/prboom/Config.in

@@ -1,5 +1,5 @@
 config BR2_PACKAGE_PRBOOM
-	bool "PrBoom"
+	bool "prboom"
 	select BR2_PACKAGE_SDL
 	select BR2_PACKAGE_SDL_MIXER
 	select BR2_PACKAGE_SDL_NET