[Dune] typo in am2cmake.py

[Martin Alkaemper]

Hello

while porting a module to cmake, we encountered a bug in the python script

dune-common/bin/am2cmake.py

Line 565
    output=open("Makefile.am'", "a")

it should be

    output=open("Makefile.am", "a")

Best
Martin Alkämper