Docs: change "_security" to "security"

[tyrannosaurus-becks]

Today I tested reading a user on Elasticsearch version 7.0.0 using the publicly released binary. I noted that past docs state the path to the security API is security, but current docs state it's _security. I tested both, and found that security is still the correct one.

This PR updates the docs to continue to reflect the path to the security API.

[bizybot]

Hi @tyrannosaurus-becks, Thank you but it seems your understanding is incorrect.

Prior to 7.0.0 version, the REST endpoint path was /_xpack/security/user.
Starting from 7.0.0 version it has been modified to /_security/user.
For backward compatibility, one can use /_xpack/security/* for old APIs. New APIs introduced in 7.0.0 version will not have '/_xpack' in the path.
Ref: #35958

Closing this as this does not seem to be applicable. Thank you.