docs: fix link in sbsa description

A trailing _ makes all the difference to the rendered link.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20210428131316.31390-1-alex.bennee@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/sbsa.rst

@@ -4,7 +4,7 @@ Arm Server Base System Architecture Reference board (``sbsa-ref``)
 While the `virt` board is a generic board platform that doesn't match
 any real hardware the `sbsa-ref` board intends to look like real
 hardware. The `Server Base System Architecture
-<https://developer.arm.com/documentation/den0029/latest>` defines a
+<https://developer.arm.com/documentation/den0029/latest>`_ defines a
 minimum base line of hardware support and importantly how the firmware
 reports that to any operating system. It is a static system that
 reports a very minimal DT to the firmware for non-discoverable