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/README.uniphier b/doc/README.uniphier
index badfacd..af746f6 100644
--- a/doc/README.uniphier
+++ b/doc/README.uniphier
@@ -336,7 +336,7 @@
 --------------------------
 
 UniPhier SoC family boot the kernel in a generic manner as described in
-doc/README.distro .
+doc/develop/distro.rst.
 
 To boot the kernel, you need to deploy necesssary components to a file
 system on one of your block devices (eMMC, NAND, USB drive, etc.).