Update graph.rst - teachers to lectures

[veto8]

The graph linking should be teachers to lectures
instead teachers to teachers
teach = school.create_edge_definition(
edge_collection='teach',
from_vertex_collections=['teachers'],
to_vertex_collections=['lectures']
)

[apetenchea]

This is somewhat debatable, see the whole example:

# Create an edge definition named "teach". This creates any missing
# collections and returns an API wrapper for "teach" edge collection.
if not school.has_edge_definition('teach'):
    teach = school.create_edge_definition(
        edge_collection='teach',
        from_vertex_collections=['teachers'],
        to_vertex_collections=['teachers']
    )

# List edge definitions.
school.edge_definitions()

# Replace the edge definition.
school.replace_edge_definition(
    edge_collection='teach',
    from_vertex_collections=['teachers'],
    to_vertex_collections=['lectures']
)

The edge definition is replaced with the correct one later on. I am not sure if this was intended to illustrate a replacement use-case, or if was a mistake.

I suggest making it obvious this time by also editing the replace_edge_definition call too with something like:

# Replace the edge definition.
    school.replace_edge_definition(
        edge_collection='teach',
        from_vertex_collections=['teachers'],
        to_vertex_collections=['new_lectures']
)

This way we've got teachers->lectures which gets replaced by teachers->new_lectures. I feel like it illustrates the point even better.

What do you think?

[apetenchea]

@veto8, thanks for the initiative!

I have added a comment to better explain the situation in #372