Fix an error in the tutorial in the Readme.md

[ThrudPrimrose]

if I call the example in the tutorial as it is, I get an error.
I call it like this:

generator.generate(output_dir, GeneratorCollection([LIBXSMM(arch), Eigen(arch)]))

The error I get is:

Deducing indices...
Generating unit tests...
Traceback (most recent call last):
  File "/home/primrose/Work/MasterThesisMaterial/yateto/playground/gen.py", line 68, in <module>
    generator.generate(output_dir, GeneratorCollection([LIBXSMM(arch), Eigen(arch)]))
  File "/home/primrose/Work/MasterThesisMaterial/yateto/yateto/generator.py", line 295, in generate
    with cpp.HeaderGuard(self._headerGuardName(namespace, self.CXXTEST_FILE_NAME.replace('.', '_'))):
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/primrose/Work/MasterThesisMaterial/yateto/yateto/generator.py", line 258, in _headerGuardName
    partlist = namespace.upper().split('::') + [fileBaseName.upper(), self.HEADER_GUARD_SUFFIX]
               ^^^^^^^^^^^^^^^
AttributeError: 'GeneratorCollection' object has no attribute 'upper'

I believe this happens that the 2nd argument in the definition is for the namespace and not for the generator collection, the following works fine:

generator.generate(outputDir=output_dir, gemm_cfg=GeneratorCollection([LIBXSMM(arch), Eigen(arch)]))

I believe it could be better to update the tutorial example.