Standardize sentry guard variable name on rust SDK docs

[TBS1996]

Core or SDK?

Platform/SDK

Which part? Which one?

docs for rust sdk

Description

In the 'getting started' when you set up a rust program, we use _guard as the name of the guard variable.
When you click set up perforamnce monitoring, the docs use _sentry. At first glance it looks like you're supposed to have two separate variables, instead of simply adding extra non-default fields.

Examples of the mismatch:

https://github.com/search?q=repo%3Agetsentry%2Fsentry-docs%20let%20_sentry%20%3D%20sentry%3A%3Ainit(sentry%3A%3AClientOptions%20%7B&type=code

https://github.com/search?q=repo%3Agetsentry%2Fsentry-docs+let+_guard+%3D+sentry%3A%3Ainit&type=code

Suggested Solution

Decide on either of the two names and keep it consistent.

I didn't make a PR in case someone is opinionated about which to choose.

[getsantry]

Routing to @getsentry/product-owners-sdks-native for triage ⏲️