doc: bump number of providers
doc: bump number of providers

It's 74 unique providers by my count from grepping the tree. Let's be
conservative and say "over 60" at this point

Closes #759

vongazman/libcloud,ZuluPro/libcloud,StackPointCloud/libcloud,SecurityCompass/libcloud,mathspace/libcloud,DimensionDataCBUSydney/libcloud,iPlantCollaborativeOpenSource/libcloud,mgogoulos/libcloud,niteoweb/libcloud,iPlantCollaborativeOpenSource/libcloud,erjohnso/libcloud,DimensionDataCBUSydney/libcloud,apache/libcloud,lochiiconnectivity/libcloud,watermelo/libcloud,Scalr/libcloud,wuyuewen/libcloud,lochiiconnectivity/libcloud,Kami/libcloud,DimensionDataCBUSydney/libcloud,samuelchong/libcloud,mathspace/libcloud,Scalr/libcloud,ByteInternet/libcloud,erjohnso/libcloud,ByteInternet/libcloud,StackPointCloud/libcloud,techhat/libcloud,Scalr/libcloud,samuelchong/libcloud,wrigri/libcloud,mgogoulos/libcloud,mathspace/libcloud,Kami/libcloud,wuyuewen/libcloud,ZuluPro/libcloud,wrigri/libcloud,apache/libcloud,wrigri/libcloud,mistio/libcloud,t-tran/libcloud,ZuluPro/libcloud,NexusIS/libcloud,ByteInternet/libcloud,supertom/libcloud,pquentin/libcloud,NexusIS/libcloud,andrewsomething/libcloud,samuelchong/libcloud,watermelo/libcloud,vongazman/libcloud,StackPointCloud/libcloud,niteoweb/libcloud,supertom/libcloud,NexusIS/libcloud,SecurityCompass/libcloud,Kami/libcloud,t-tran/libcloud,t-tran/libcloud,apache/libcloud,pquentin/libcloud,andrewsomething/libcloud,illfelder/libcloud,niteoweb/libcloud,illfelder/libcloud,techhat/libcloud,vongazman/libcloud,illfelder/libcloud,lochiiconnectivity/libcloud,techhat/libcloud,wuyuewen/libcloud,mistio/libcloud,iPlantCollaborativeOpenSource/libcloud,mistio/libcloud,SecurityCompass/libcloud,supertom/libcloud,erjohnso/libcloud,mgogoulos/libcloud,watermelo/libcloud,pquentin/libcloud,andrewsomething/libcloud