Fix documentation by explicitly dropping connection

[yerke]

Fix documentation by explicitly dropping connection

Everywhere in code we use drop(conn);. I guess docs were left out when this change happened.

Without this rustc will complain:

  --> tests/health_check.rs:52:5
   |
52 |     conn;
   |     ^^^^^ help: use `drop` to clarify the intent: `drop(conn);`
   |
   = note: `#[warn(path_statements)]` on by default

[yerke]

I see that TestMariaDb v101 CI job already fails on master branch.

[blackbeam]

Hi. Thanks for PR.
CI failure is unrelated and will be fixed in #154, so I'm merging this.