Typo in "tessellation" configuration variable name

There were some instances of "tessellation" spelled "tesselation" that were fixed in the 2.4 branch.  The configuration variable was left spelled wrong because changing it would be best if there was a migration from the old to the new setting. We'll just do that for 2.5/2.6, so correct this in the master branch. A migration should be added when migrating from the 2.4 config to the 2.5/2.6 config.