docs: acme: Fix typo

Changed files
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nixos
modules
security
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nixos/modules/security/acme.xml
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<para>
This is useful if you want to generate a wildcard certificate, since
ACME servers will only hand out wildcard certs over DNS validation.
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There a number of supported DNS providers and servers you can utilise,
+
There are a number of supported DNS providers and servers you can utilise,
see the <link xlink:href="https://go-acme.github.io/lego/dns/">lego docs</link>
for provider/server specific configuration values. For the sake of these
docs, we will provide a fully self-hosted example using bind.