Documentation/tcp: Fix an obvious typo

Yep, my VIM spellchecker is not good enough for typos like this one.

Fixes: 7fe0e38 ("Documentation/tcp: Add TCP-AO documentation")
Cc: Jonathan Corbet <[email protected]>
Cc: [email protected]
Reported-by: Markus Elfring <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Dmitry Safonov <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>

Author: 0x7f454c46

Documentation/networking/tcp_ao.rst

@@ -99,7 +99,7 @@ also [6.1]::
    when it is no longer considered permitted.
 
 Linux TCP-AO will try its best to prevent you from removing a key that's
-being used, considering it a key management failure. But sine keeping
+being used, considering it a key management failure. But since keeping
 an outdated key may become a security issue and as a peer may
 unintentionally prevent the removal of an old key by always setting
 it as RNextKeyID - a forced key removal mechanism is provided, where