Change OPAM to opam

[christinerose]

I was asked to open this PR to change this.

Issue Description

Changing OPAM to opam

Fixes # (issue)

Changed OPAM to opam

Please describe the changes that you made.

• Please check if the PR fulfills these requirements

• ❗ If the PR changes a markdown document in site/, a comment was added in https://github.com/ocaml/ood/issues/52 with a link to the PR
• PR is descriptively titled and links the original issue above
• Before/after screenshots (if this is a layout change)
• Details of which platforms the change was tested on (if this is a browser-specific change)
• Context for what motivated the change (if this is a change to some content)
  Thomas confirmed the proper way to write opam was in all lower case, so a supervisor asked me to open a PR to change this when I came across it this morning.