This change fixes automake bug#25413.
* doc/automake.texi (Serial Test Harness): Fix typo.
The serial test harness is enabled by the Automake option
@option{serial-tests}. It operates by simply running the tests serially,
one at the time, without any I/O redirection. It's up to the user to
-implement logging of tests' output, if that's requited or desired.
+implement logging of tests' output, if that's required or desired.
For historical and implementation reasons, the @code{AM_TESTS_ENVIRONMENT}
variable is @emph{not} supported by this harness (it will be silently