dnsmint: honour DNSMint_Api instead of overwriting it

The assignment was unconditional, so exporting DNSMint_Api did nothing - the
plugin reset it to the default every time it was sourced. Every neighbouring
plugin with an _Api variable treats it as an override; this one only looked
like it did.

Found while writing the dnsapi2 entry: the sentence documenting the override
would have been false.

dnsapi/dns_dnsmint.sh

@@ -12,7 +12,7 @@ Issues: github.com/dnsmint/acme.sh
 Author: DNSMint
 '
 
-DNSMint_Api="https://dnsmint.com/api"
+DNSMint_Api="${DNSMint_Api:-https://dnsmint.com/api}"
 
 ########  Public functions #####################