CRM_Utils_Hook::alterMailer -- Fix typo

CRM_Utils_Hook::alterMailer -- Fix typo

There was a discrepancy in the naming of this hook:

 * The stub function was named `alterMail`. Internal code therefore referred to `alterMail`.
 * The hook invocation fires as `alterMailer`. Downstream code therefore referred to `alterMailer`.

This patch standardizes on `alterMailer` to preserve the downstream
interface.  It leaves `::alterMail()` as a deprecated alias in case there's
some unexpected external use-case.