docs: remove missing link from contributing.md

[cddm-gh]

remove the link referencing the missing developer.md file

resolves #1300

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Other... Please describe: docs(contributing.md)

What is the current behavior?

Issue Number: #1300

What is the new behavior?

The file doesn't refer to a missing DEVELOPER.md file anymore

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[kamilmysliwiec]

LGTM