Fix dialyzer error in the sign_v4_event/7 spec

[mikpe]

The returned headers are custom and do not match the headers() type.

Fixes the following warning when running rebar3 dialyzer on this repo:

===> Analyzing 2 files with _build/default/rebar3_27.3_plt...

src/aws_signature.erl
Line 132 Column 2: Invalid type specification for function aws_signature:sign_v4_event/7. The success typing is (bina\
ry(),binary(),binary(),{{non_neg_integer(),1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12,1..255},{byte(),byte(),by\
te()}},binary(),binary(),binary()) -> {[{<<_:40,_:_*88>>,binary() | {_,_},'byte_array' | 'timestamp'},...],binary()} \
but the spec is (SecretAccessKey,Region,Service,DateTime,PriorSignature,HeaderString,Body) -> {Headers,Signature} whe\
n SecretAccessKey :: binary(), Region :: binary(), Service :: binary(), DateTime :: calendar:datetime(), PriorSignatu\
re :: binary(), HeaderString :: binary(), Body :: binary(), Headers :: headers(), Signature :: binary()
===> Warnings written to _build/default/27.3.dialyzer_warnings
===> Warnings occurred running dialyzer: 1

[onno-vos-dev]

Thank you! 🙇 Let me know if you want a release but I know you have another PR coming soon so I'll wait unless you want one 👍

[mikpe]

Thanks, but I don't need a release or a tag at this time.