Multisite Sub Directory Install Problem: Update bedrock-with-valet.md

[eYinka]

Fixes the problem with sub-directly installs not working. Essentially, setting SUBDOMAIN_INSTALL to true makes WP assume it's a sub-domain setup at all times. Therefore, we make it false in the case of sub directories. Problem solved!

Also, main documentation says multisite-url-fixer is optional for sub-directory instances. Add optional to the last line.

[swalkinshaw]

Thanks @eYinka