fix: write directory instead of folder

[ashish0kumar]

fixes #2569

This PR replaces all occurrences of "folder" with "directory" in the following files:

• [X] 6 data/tutorials/getting-started/1_02_your_first_ocaml_program.md
• [X] 1 data/tutorials/language/1ms_00_modules.md
• [X] 1 data/tutorials/language/1ms_01_functors.md
• [X] 13 data/tutorials/language/1ms_02_dune.md
• [X] 2 data/tutorials/platform/0_09_opam_path.md
• [X] 1 data/tutorials/platform/2_08_odoc.md

[cuihtlauac]

Thanks @ashish0kumar