Update index.md
Update index.md

Correct link to RNA-seq galaxy 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