performance: fix innodb configuration variable name

The variable is innodb_doublewrite, while the command-line option is --skip-innodb-doublewrite

See https://mariadb.com/docs/server/ref/mdb/system-variables/innodb_doublewrite/

docs/Performance and Tuning/Workload Tuning.rst

@@ -626,7 +626,7 @@ writes and leave recordsize=128K on the log files. Set
 caching [#mysql_basic]_.
 Set ``logbias=throughput`` on the data to stop ZIL from writing twice.
 
-Set ``skip-innodb_doublewrite`` in my.cnf to prevent innodb from writing
+Set ``innodb_doublewrite=0`` in my.cnf to prevent innodb from writing
 twice. The double writes are a data integrity feature meant to protect
 against corruption from partially-written records, but those are not
 possible on ZFS. It should be noted that `Percona’s