bigdecimal/math.rb: error message in BigMath#PI

* ext/bigdecimal/lib/bigdecimal/math.rb (BigMath#PI): change error
  message about zero or negative precision for clarity and
  consistency with other methods.  [GH-644]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46600 b2dd03c8-39d4-4d8f-98ff-823fe69b080e