package/mupdf: update homepage URL

The current homepage URL leads to an HTTP 404 error.

Fix it by using the homepage URL currently mentioned in mupdf's git
repository.

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit d1ea9a64e68960be7737e641c98c6990a06e72c0)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

package/mupdf/Config.in

@@ -14,7 +14,7 @@ config BR2_PACKAGE_MUPDF
 	help
 	  MuPDF is a lightweight PDF, XPS, and E-book viewer.
 
-	  https://www.mupdf.com/index.html
+	  https://www.mupdf.com/
 
 comment "mupdf needs a toolchain w/ C++, gcc >= 4.9"
 	depends on BR2_TOOLCHAIN_HAS_SYNC_4