glasgow_exts.rst: fix code block
glasgow_exts.rst: fix code block

Detected by sphinx as:
    ERROR: Unexpected indentation.

Signed-off-by: Sergei Trofimovich <df7d2cee2e7ececeb2368216a0f9d0537fd32149@google.com>

vTurbine/ghc,nushio3/ghc,oldmanmike/ghc,sgillespie/ghc,gridaphobe/ghc,gridaphobe/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,ezyang/ghc,vikraman/ghc,mcschroeder/ghc,mcschroeder/ghc,sdiehl/ghc,gridaphobe/ghc,vTurbine/ghc,GaloisInc/halvm-ghc,mettekou/ghc,shlevy/ghc,GaloisInc/halvm-ghc,ezyang/ghc,mcschroeder/ghc,tjakway/ghcjvm,oldmanmike/ghc,mettekou/ghc,tjakway/ghcjvm,shlevy/ghc,snoyberg/ghc,vikraman/ghc,mcschroeder/ghc,sgillespie/ghc,sdiehl/ghc,mettekou/ghc,sgillespie/ghc,vikraman/ghc,tjakway/ghcjvm,tjakway/ghcjvm,ezyang/ghc,snoyberg/ghc,olsner/ghc,oldmanmike/ghc,snoyberg/ghc,sgillespie/ghc,ezyang/ghc,nushio3/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,shlevy/ghc,nushio3/ghc,nushio3/ghc,olsner/ghc,vTurbine/ghc,nushio3/ghc,tjakway/ghcjvm,olsner/ghc,vikraman/ghc,vikraman/ghc,oldmanmike/ghc,GaloisInc/halvm-ghc,sgillespie/ghc,vTurbine/ghc,gridaphobe/ghc,snoyberg/ghc,mcschroeder/ghc,shlevy/ghc,gridaphobe/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,vTurbine/ghc,nushio3/ghc,vTurbine/ghc,mcschroeder/ghc,ezyang/ghc,sdiehl/ghc,GaloisInc/halvm-ghc,oldmanmike/ghc,ezyang/ghc,GaloisInc/halvm-ghc,shlevy/ghc,vikraman/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,snoyberg/ghc,mettekou/ghc,tjakway/ghcjvm,tjakway/ghcjvm,mcschroeder/ghc,mettekou/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,mettekou/ghc,ezyang/ghc,oldmanmike/ghc,vTurbine/ghc,gridaphobe/ghc,sgillespie/ghc,vikraman/ghc,nushio3/ghc,GaloisInc/halvm-ghc,oldmanmike/ghc,sdiehl/ghc