Update core_concepts.rst
Update core_concepts.rst

fixed typos
pombredanne/viewflow,codingjoe/viewflow,pombredanne/viewflow,codingjoe/viewflow,viewflow/viewflow,viewflow/viewflow,ribeiro-ucl/viewflow,viewflow/viewflow,ribeiro-ucl/viewflow,codingjoe/viewflow,ribeiro-ucl/viewflow