dev-manual: Fixed variable link to DEFAULTTUNE variable

Fixed [YOCTO #12458]

(From yocto-docs rev: 161d7ec7c81db93120af11b07162b36785614302)

Signed-off-by: Kristi Rifenbark <kristi@buzzcollectivemarketing.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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

@@ -7236,7 +7236,7 @@ Some notes from Cal:
             you much of a performance difference across the other systems
             as compared to using a more general tuning across all the builds
             (e.g. setting
-            <ulink url='var-DEFAULTTUNE'><filename>DEFAULTTUNE</filename></ulink>
+            <ulink url='&YOCTO_DOCS_REF_URL;#var-DEFAULTTUNE'><filename>DEFAULTTUNE</filename></ulink>
             specifically for each machine's build).
             Rather than "max out" each build's tunings, you can take steps that
             cause the OpenEmbedded build system to reuse software across the