RFR: JDK-8248687: JPackage test extension misspelled "extention"

[Andy Herrick]

please review trivial jpackage fix to issue [1] at [2]

[1] https://bugs.openjdk.java.net/browse/JDK-8248687

[2] http://cr.openjdk.java.net/~herrick/8248687/webrev.01/

/Andy