BUG/MINOR: checks: typo in an email-alert error message

When the email alert message couldn't be formatted, the logged error message
said the contrary.

This fix must be backported to 1.6.

src/checks.c

@@ -3342,7 +3342,7 @@ void send_email_alert(struct server *s, int level, const char *format, ...)
 	va_end(argp);
 
 	if (len < 0) {
-		Alert("Email alert [%s] could format message\n", p->id);
+		Alert("Email alert [%s] could not format message\n", p->id);
 		return;
 	}