dev-manual: Fixed bad link

Link to metadata term was not going to the ref-manual.

(From yocto-docs rev: 7a116ece403cb88977dfcce3006e2e1bb52b5df0)

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

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -7059,7 +7059,7 @@ Some notes from Cal:
 
                 <para>
                     As mentioned, attempting to maintain revision numbers in the
-                    <ulink url='&YOCTO_DOCS_DEV_URL;#metadata'>Metadata</ulink>
+                    <ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>
                     is error prone, inaccurate, and causes problems for people
                     submitting recipes.
                     Conversely, the PR Service automatically generates