Update r-tidyverse-2017-12-15.md
Update r-tidyverse-2017-12-15.md

Remove sign-up link for intermediate R course
sheffield-bioinformatics-core/sheffield-bioinformatics-core.github.io,sheffield-bioinformatics-core/sheffield-bioinformatics-core.github.io,sheffield-bioinformatics-core/sheffield-bioinformatics-core.github.io,sheffield-bioinformatics-core/sheffield-bioinformatics-core.github.io