Create `salmon_outputs` directory in Data Exploration

[tavareshugo]

Some participants sometimes don't create the salmon_outputs directory from the first session and this then breaks the saveRDS in Data Exploration section.

We can replace this line of code with:

# create directory if it doesn't exist
dir.create("salmon_outputs", showWarnings = FALSE)
# save txi object
saveRDS(txi, file = "salmon_outputs/txi.rds")

(sorry I don't have the data handy to push the fix quickly)

[tavareshugo]

In exercise 2 of Salmon section -- do not create directory (this was already created by us).
Mention that we created with mkdir salmon_outputs ourselves before the course.

Needs fixing in the practical materials and in the solutions markdown.