message: docs: fix typo, list subtrees in priority gudiance
diff: @@ -15,7 +15,7 @@ request review from someone else. This will also reassign the PR.
  We never merge PRs directly. Instead, we use bors. A qualified
  reviewer with bors privileges (e.g., a [compiler
  -contributor](./membership.md) will leave a comment like `@bors r+`.
  +contributor](./membership.md)) will leave a comment like `@bors r+`.
  This indicates that they approve the PR.
  People with bors privileges may also leave a `@bors r=username`
  @@ -83,7 +83,7 @@ The following is some guidance for setting priorities:
  - P-high issue fixes
  - Toolstate fixes
  - Beta-nominated PRs
  - - Submodule updates
  + - Submodule/Subtree updates
  - 5+
  - P-critical issue fixes
  - 10+
  