sdk-manual: Converted external links to internal links.

Two links that should have been to different places were overlooked.
I fixed these.

(From yocto-docs rev: a1681d065812546a573ed778141289a6b476cecd)

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

documentation/sdk-manual/sdk-extensible.xml

@@ -15,7 +15,7 @@
         to an image, modify the source for an existing component, test
         changes on the target hardware, and ease integration into the rest of
         the
-        <ulink url='&YOCTO_DOCS_DEV_URL;#build-system-term'>OpenEmbedded build system</ulink>.
+        <ulink url='&YOCTO_DOCS_REF_URL;#build-system-term'>OpenEmbedded build system</ulink>.
         <note>
             For a side-by-side comparison of main features supported for an
             extensible SDK as compared to a standard SDK, see the
@@ -306,7 +306,7 @@
                 The <filename>devtool add</filename> command generates
                 a new recipe based on existing source code.
                 This command takes advantage of the
-                <ulink url='&YOCTO_DOCS_DEV_URL;#devtool-the-workspace-layer-structure'>workspace</ulink>
+                <ulink url='&YOCTO_DOCS_REF_URL;#devtool-the-workspace-layer-structure'>workspace</ulink>
                 layer that many <filename>devtool</filename> commands
                 use.
                 The command is flexible enough to allow you to extract source