GITBOOK-52: change request with no subject merged in GitBook

Author: Moloejoe

pgml-docs/docs/guides/getting-started/sign-up.md

@@ -5,6 +5,7 @@
 1. Go to [https://postgresml.org/signup](https://postgresml.org/signup)
 2. Sign up using your email or using Google or Github authentication
 3. Login using your account
+4. [data-pre-processing.md](../machine-learning/supervised-learning/data-pre-processing.md "mention")