Documentation/applying-patches.txt: fix a bad external link

We can't use :ref: for external links.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/applying-patches.txt

@@ -427,7 +427,7 @@ The -mm patches are experimental patches released by Andrew Morton.
 
 In the past, -mm tree were used to also test subsystem patches, but this
 function is now done via the
-:ref:`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
+`linux-next <https://www.kernel.org/doc/man-pages/linux-next.html>`
 tree. The Subsystem maintainers push their patches first to linux-next,
 and, during the merge window, sends them directly to Linus.