fix maxVtx definition in fillDescription method (needed for confDB)

while testing a new HLT configuration made w/ confDB [via hltIntegrationTest]
I found the following error message

22-May-2014 18:57:01 CEST  Initiating request to open file root://eoscms//eos/cms/store/user/tosi/trackingHLT/mc/TTbar25nsHighPU/TT_Tune4C_13TeV-pythia8-tauola_PU40bx25_POSTLS162_V2-v1_1.root?svcClass=default
22-May-2014 18:57:05 CEST  Successfully opened file root://eoscms//eos/cms/store/user/tosi/trackingHLT/mc/TTbar25nsHighPU/TT_Tune4C_13TeV-pythia8-tauola_PU40bx25_POSTLS162_V2-v1_1.root?svcClass=default
22-May-2014 18:57:18 CEST  Closed file root://eoscms//eos/cms/store/user/tosi/trackingHLT/mc/TTbar25nsHighPU/TT_Tune4C_13TeV-pythia8-tauola_PU40bx25_POSTLS162_V2-v1_1.root?svcClass=default
----- Begin Fatal Exception 22-May-2014 18:57:18 CEST-----------------------
An exception of category 'Configuration' occurred while
   [0] Constructing the EventProcessor
   [1] Constructing module: class=PixelVertexCollectionTrimmer label='hltPixelVertices'
Exception Message:
ValueError type of maxVtx is expected to be unsigned int but declared as int32
----- End Fatal Exception -------------------------------------------------