sdk-manual: add missing quoting around "devtool upgrade"

(From yocto-docs rev: 8863967d1e4e4a5b8014bf55302752419352c25d)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sdk-manual/extensible.rst

@@ -603,7 +603,7 @@ counterparts.
 .. note::
 
    Several methods exist by which you can upgrade recipes -
-   devtool upgrade
+   ``devtool upgrade``
    happens to be one. You can read about all the methods by which you
    can upgrade recipes in the
    :ref:`dev-manual/common-tasks:upgrading recipes` section