collection3: Fix inappropriate usage of CGI:param

collection3: Fix inappropriate usage of CGI:param

When using collection3 as a CGI, the following error is sent to logs
repeatedly.

  FastCGI sent in stderr: "CGI::param called in list context from
  /usr/share/doc/collectd-core/examples/collection3/lib/Collectd/Graph/Common.pm
  line 529, this can lead to vulnerabilities. See the warning in
  "Fetching the value or values of a single named parameter" at
  /usr/share/perl5/CGI.pm line 412"

This is caused inappropriate usage of param(), it should be handled as
a scalar or should be treated by multi_param() explicitly.

Signed-off-by: Kentaro Hayashi <kenhys@gmail.com>