Fix multisite search-replace example

[kevinfodness]

Adds the --network flag and the wp_site table to the example about how to prepare a multisite for local development:

• Without the --network flag, the wildcard in the example for performing a search-replace across all options tables for all subsites fails to match subsite options tables and only matches the main options table (e.g., wp_options).
• Without the wp_site table, the search-replace for domains is incomplete if targeting the primary site.

[kevinfodness]

Thank you! This was on my to-do list but I hadn't gotten to it yet.