[CBR-377] Fix insidious bug with port binding
[CBR-377] Fix insidious bug with port binding

This fixes an insidious bug which was causing the backend server to be
spawn on the same port of the documentation, causing an 'Address already
in use' failure.

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl