(maybe) Fixes a typo in the `rspamd` docs

[reind33r]

Currently, to add rspamd, the docs suggest to

add rspamd check with default configuration into appropriate check block (probably in local_routing):

checks {
   ...
   rspamd
}```

Adding this block in local_routing results in a failure to start, because in this context only check is valid.

(I see that checks exist at top-level, but if we mention local_routing and appropriate check block, it makes more sense to me that the code block is also check)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 52.65%. Comparing base (798c411) to head (bb2c0c9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #682 +/- ## ========================================== - Coverage 52.68% 52.65% -0.03% ========================================== Files 129 129 Lines 13369 13369 ========================================== - Hits 7043 7039 -4 - Misses 5660 5663 +3 - Partials 666 667 +1 ``` | [Flag](https://app.codecov.io/gh/foxcpp/maddy/pull/682/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Max+Mazurov) | Coverage Δ | | |---|---|---| | [integration](https://app.codecov.io/gh/foxcpp/maddy/pull/682/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Max+Mazurov) | `34.36% <ø> (+0.09%)` | :arrow_up: | | [unit](https://app.codecov.io/gh/foxcpp/maddy/pull/682/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Max+Mazurov) | `52.16% <ø> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Max+Mazurov#carryforward-flags-in-the-pull-request-comment) to find out more.

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[foxcpp]

It is indeed a typo.

checks and modifiers are names of group modules that allow defining reusable configurations for check and modify directives correspondingly:

checks inbound_filters {
        dkim
}

modifiers inbound_mutation {
        replace_rcpt regexp "asd" "asd"
}

smtp tcp://127.0.0.1:2525 {
        check &inbound_filters
        modify &inbound_mutation
        reject
}

[foxcpp]

Thanks!