doc: fix references to distro documentation

Commit 37c5195dfcd157 ("doc: Move distro boot doc to rST") renamed
doc/README.distro to doc/develop/distro.rst.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Soeren Moch <smoch@web.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/doc/board/emulation/qemu-x86.rst b/doc/board/emulation/qemu-x86.rst
index db842f2..e7dd4e9 100644
--- a/doc/board/emulation/qemu-x86.rst
+++ b/doc/board/emulation/qemu-x86.rst
@@ -57,7 +57,8 @@
 U-Boot uses 'distro_bootcmd' by default when booting on x86 QEMU. This tries to
 load a boot script, kernel, and ramdisk from several different interfaces. For
 the default boot order, see 'qemu-x86.h'. For more information, see
-'README.distro'. Most Linux distros can be booted by writing a uboot script.
+'doc/develop/distro.rst'. Most Linux distros can be booted by writing a uboot
+script.
 For example, Debian (stretch) can be booted by creating a script file named
 'boot.txt' with the contents::