📝 docs: fix path string in fullstack guide

[byhow]

Closes #364

For minor file path adjustments.

[yoshi-monster]

Hello!

This is incorrect: The path should be src/shared/groceries.gleam inside the shared project. This section is about the shared project, and it says to cd into it one code block above. The full path relative to the monorepo root would be shared/src/shared/groceries.gleam.

However, this is something that repeatedly trips people up reading that guide. It would be great to figure out a way to communicate that in a way that makes people not think it's a typo.

[byhow]

@yoshi-monster Ah got it, thanks for the timely response! I was a bit confused at first because I didn't find the shared directory (and seeing another shared.gleam file) but now I see that it is another mkdir inside shared/src. I updated the languages in the documentation to more explicitly point out that it is a brand new file and directory. Hopefully this clears things up.

[hayleigh-dot-dev]

I think the deeper problem here is the guide assumes the reader thinks like the shell, and so when we say cd shared we are assuming the reader has also mentally "changed directory."

These guides could definitely do with some love, thank you!