Replace "External accounts" with "Externally-owned accounts" for accuracy

[codersjj]

Correct terminology to align with Ethereum's official documentation.
In addition, this is also consistent with the content on Wikipedia.

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[nikola-matic]

There are a total of four instances of 'external accounts', whereas you've only changed one.

introduction-to-smart-contracts.rst:of a keypair belonging to :ref:`external accounts<accounts>`.
introduction-to-smart-contracts.rst:address space: **External accounts** that are controlled by
introduction-to-smart-contracts.rst:The address of an external account is determined from
security-considerations.rst:- Neither contracts nor "external accounts" are currently able to prevent someone from sending them Ether.

[codersjj]

There are a total of four instances of 'external accounts', whereas you've only changed one.

introduction-to-smart-contracts.rst:of a keypair belonging to :ref:`external accounts<accounts>`.
introduction-to-smart-contracts.rst:address space: **External accounts** that are controlled by
introduction-to-smart-contracts.rst:The address of an external account is determined from
security-considerations.rst:- Neither contracts nor "external accounts" are currently able to prevent someone from sending them Ether.

You're right! I missed the other instances. I'll update all four. Thanks! 🙌