Fixed incorrect isModified method name causing error

Whoops, my bad.
Blog: Wrapped CMS fields in beforeUpdateCMSFields to allow for customisation
Merge pull request #21 from purplespider/beforeupdatecmsfields

Blog: Wrapped CMS fields in beforeUpdateCMSFields
Merge pull request #20 from purplespider/patch-1

Fixed incorrect isModified method name causing error