Update readme.md

Fix the URL

readme.md

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 <img align="left" src=bin/dbatools.png alt="dbatools logo">  dbatools is PowerShell module that you may think of like a command-line SQL Server Management Studio. The project initially started out as just `Start-SqlMigration.ps1`, but has now grown into a collection of [over 500 commands](https://dbatools.io/commands) that help automate SQL Server tasks and encourage best practices.
 
-Want to contribute to the project? We'd love to have you! Visit our [contributing.md](contributing.md) for a jump start.
+Want to contribute to the project? We'd love to have you! Visit our [contributing.md](CONTRIBUTING.md) for a jump start.
 
 Want to say thanks? Click the star at the top of the page 🌟