(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

@@            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	Coverage Δ
integration	34.36% <ø> (+0.09%)	:arrow_up:
unit	52.16% <ø> (-0.05%)	:arrow_down:

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