Update messages.po

[oliof]

tiny grammar fix.

• Your changes are not possible to do through a plugin and relevant
  to a large audience (ideally all users of OctoPrint)
• If your changes are large or otherwise disruptive: You have
  made sure your changes don't interfere with current development by
  talking it through with the maintainers, e.g. through a
  Brainstorming ticket
• Your PR targets OctoPrint's devel branch if it's a completely
  new feature, or maintenance if it's a bug fix or improvement of
  existing functionality for the current stable version (no PRs
  against master or anything else please)
• Your PR was opened from a custom branch on your repository
  (no PRs from your version of master, maintenance or devel please),
  e.g. dev/my_new_feature or fix/my_bugfix
• Your PR only contains relevant changes: no unrelated files,
  no dead code, ideally only one commit - rebase and squash your PR
  if necessary!
• Your changes follow the existing coding style
• If your changes include style sheets: You have modified the
  .less source files, not the .css files (those are generated with
  lessc)
• You have tested your changes (please state how!) - ideally you
  have added unit tests
• You have run the existing unit tests against your changes and
  nothing broke
• You have added yourself to the AUTHORS.md file :)

What does this PR do and why is it necessary?

How was it tested? How can it be tested by the reviewer?

Any background context you want to provide?

What are the relevant tickets if any?

Screenshots (if appropriate)

Further notes

[foosel]

Thank you!