Fix minor typo in UsingFeatureExtraction.Rmd

[rbcavanaugh]

Hi - Working through the very helpful UsingFeatureExtraction tutorial.

I think line 436 should read

• covariateId identifies the covariate, and definitions of covariates can be found in the covariateData$covariateRef object

not

• covariateId identifies the covariate, and definitions of covariates can be found in the cohortData$covariateRef object.

I can't find reference to a cohortData object in the vignette and covariateData$covariateRef appears to hold the referenced information. Apologies if this is wrong (and if so, please let me know where I Can find the cohortData object). Happy to create a pull request. Rob

[anthonysena]

Thanks @rbcavanaugh - I believe you are correct! If you could create a PR, I'll work to get that into the next release. Thank you!