Fix admin-api overview: update access key instructions

[athanor-omgwinning]

The bottom section of the Admin API Overview page needs two changes:

1. Remove the line 'Open the Alchemy Dashboard' (step 1)
2. Edit step 2 to say 'Go to the security tab in the alchemy dashboard'

File to edit: content/admin-api/overview.mdx

Current content:

1. Open the [Alchemy Dashboard](https://dashboard.alchemy.com/)
2. Go to the [Security Tab](https://dashboard.alchemy.com/settings/security)
3. Create a new access key
4. Enable App Management with Read & Write permissions

Desired content:

1. Go to the security tab in the alchemy dashboard
2. Create a new access key
3. Enable App Management with Read & Write permissions

Requested-by: https://alchemyinsights.slack.com/archives/C0AKT9ZDWBE/p1774824795035759