Edit Embedded Wallets Flutter quickstart

[bgravenorst]

See the Social Logins quickstart:
https://docs.metamask.io/quickstart/?product=EMBEDDED_WALLETS&framework=FLUTTER&stepIndex=0

Update the content to be consistent with the other quickstarts. or example, React

[m4sterbunny]

Notes:

• React doesn't have prerequisites while others such as Android do /Users/harriebickle/GitHub/Consensys/metamask-docs/src/pages/quickstart/builder/embedded-wallets/android/stepContent/requirements.mdx

The quickstart download contains the prerequisites, will need to see why diff with Android