docs: nf_flowtable: fix typo.

"mailined" should be "mainlined."
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Documentation/networking/nf_flowtable.rst

@@ -109,7 +109,7 @@ More reading
 This documentation is based on the LWN.net articles [1]_\ [2]_. Rafal Milecki
 also made a very complete and comprehensive summary called "A state of network
 acceleration" that describes how things were before this infrastructure was
-mailined [3]_ and it also makes a rough summary of this work [4]_.
+mainlined [3]_ and it also makes a rough summary of this work [4]_.
 
 .. [1] https://lwn.net/Articles/738214/
 .. [2] https://lwn.net/Articles/742164/