Fix comments

Author: HyukjinKwon

appveyor.yml

@@ -43,7 +43,7 @@ install:
   - ps: .\dev\appveyor-install-dependencies.ps1
   # Required package for R unit tests
   - cmd: R -e "install.packages(c('knitr', 'rmarkdown', 'devtools', 'e1071', 'survival'), repos='http://cran.us.r-project.org')"
-  # Here, we use fixed testthat version. See SPARK-22817.
+  # Here, we use the fixed version of testthat. For more details, please see SPARK-22817.
   - cmd: R -e "devtools::install_version('testthat', version = '1.0.2', repos='http://cran.us.r-project.org')"
   - cmd: R -e "packageVersion('knitr'); packageVersion('rmarkdown'); packageVersion('testthat'); packageVersion('e1071'); packageVersion('survival')"