doc: fix link to AESNI mb external library

[ upstream commit a0eb7f2 ]

Add missing _.

Fixes: 2977a13 ("doc: fix AESNI_MB guide")

Signed-off-by: David Marchand <david.marchand@redhat.com>

doc/guides/cryptodevs/aesni_mb.rst

@@ -62,7 +62,7 @@ To build DPDK with the AESNI_MB_PMD the user is required to download the multi-b
 library from `here <https://github.com/01org/intel-ipsec-mb>`_
 and compile it on their user system before building DPDK.
 The latest version of the library supported by this PMD is v0.51, which
-can be downloaded from `<https://github.com/01org/intel-ipsec-mb/archive/v0.51.zip>`.
+can be downloaded from `<https://github.com/01org/intel-ipsec-mb/archive/v0.51.zip>`_.
 
 .. code-block:: console