Fix typo in heartbeat/MailTo that behaves differently when /bin/sh is

bash vs. dash by using curly-braces instead of square brackets to define the variable.

Without this either "0" or "$[OCF_RESKEY_email_default]" is embedded in the ocf_heartbeat_MailTo.7 manpage depending on the shell, neither of which are probably the intended value.

This impacts the reproducibility of the build:

https://reproducible-builds.org/docs/