Fix "from package" Xenial install instructions
Fix "from package" Xenial install instructions

[skip ci]

kulibali/ponyc,Praetonus/ponyc,dipinhora/ponyc,kulibali/ponyc,kulibali/ponyc,Theodus/ponyc,sgebbie/ponyc,Praetonus/ponyc,CausalityLtd/ponyc,Theodus/ponyc,dipinhora/ponyc,cquinn/ponyc,cquinn/ponyc,Perelandric/ponyc,Praetonus/ponyc,dipinhora/ponyc,ponylang/ponyc,Theodus/ponyc,Perelandric/ponyc,jemc/ponyc,Perelandric/ponyc,sgebbie/ponyc,boemmels/ponyc,sgebbie/ponyc,sgebbie/ponyc,Perelandric/ponyc,boemmels/ponyc,boemmels/ponyc,cquinn/ponyc,ponylang/ponyc,kulibali/ponyc,Theodus/ponyc,sgebbie/ponyc,cquinn/ponyc,Praetonus/ponyc,jemc/ponyc,boemmels/ponyc,boemmels/ponyc,Theodus/ponyc,Perelandric/ponyc,CausalityLtd/ponyc,jemc/ponyc,ponylang/ponyc,CausalityLtd/ponyc