bsp-guide: Fixed broken link to Raspberry Pi README.md file.

(From yocto-docs rev: f3d4710eef845ef356072645597b3794cb19d08a)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/bsp-guide/bsp.xml

@@ -1338,7 +1338,7 @@
                     <filename>meta-</filename><replaceable>bsp_root_name</replaceable>
                     directory.
                     See the
-                    <ulink url='&YOCTO_GIT_URL;/cgit.cgi/meta-raspberrypi/tree/README'><filename>README</filename></ulink>
+                    <ulink url='&YOCTO_GIT_URL;/cgit.cgi/meta-raspberrypi/tree/README.md'><filename>README.md</filename></ulink>
                     file for the Raspberry Pi BSP in the
                     <filename>meta-raspberrypi</filename> BSP layer
                     as an example.</para>