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.gpt b/doc/README.gpt
index cc2a1b7..386ac2e 100644
--- a/doc/README.gpt
+++ b/doc/README.gpt
@@ -177,7 +177,7 @@
      "name=u-boot,size=60MiB;name=boot,size=60Mib,bootable;name=rootfs,size=0"
    It can be used to locate bootable disks with command
    "part list <interface> <dev> -bootable <varname>",
-   please check out doc/README.distro for use.
+   please check out doc/develop/distro.rst for use.
 
 2. Define 'CONFIG_EFI_PARTITION' and 'CONFIG_CMD_GPT'