Config.in.legacy: fix typo on BR2_PACKAGE_MEDIAART_BACKEND_QT

Currently using this minimal .config:
 BR2_PACKAGE_MEDIAART_BACKEND_GDK_PIXBUF=y
triggers:
     *** Legacy options removed in 2018.05 ***
 [ ] libmediaart none backend option renamed (NEW)
 [*] libmediaart gdk-pixbuf backend option renamed
 [*] libmediaart qt backend option renamed

Commit "8553b39887 libmediaart: rename options to have proper prefix",
part of the 2018.05 release had a copy&paste error.

As stated in the beginning of the file:
 The oldest symbols will be removed again after about two years.
But while we carry these legacy symbols, let's fix the typo.

Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in.legacy

@@ -2700,7 +2700,7 @@ config BR2_PACKAGE_MEDIAART_BACKEND_GDK_PIXBUF
 	  BR2_PACKAGE_MEDIAART_BACKEND_GDK_PIXBUF has been renamed to
 	  BR2_PACKAGE_LIBMEDIAART_BACKEND_GDK_PIXBUF
 
-config BR2_PACKAGE_MEDIAART_BACKEND_GDK_PIXBUF
+config BR2_PACKAGE_MEDIAART_BACKEND_QT
 	bool "libmediaart qt backend option renamed"
 	select BR2_LEGACY
 	help