Fix docstrings: `Nodes.get`, `Edges.get`

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (95fa737) 100.00% compared to head (7bc54c8) 100.00%.

:exclamation: Current head 7bc54c8 differs from pull request most recent head 90447e5. Consider uploading reports for the commit 90447e5 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #239 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 30 30 Lines 2212 2212 ========================================= Hits 2212 2212 ``` | [Flag](https://app.codecov.io/gh/BlueBrain/snap/pull/239/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [pytest](https://app.codecov.io/gh/BlueBrain/snap/pull/239/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | `100.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/BlueBrain/snap/pull/239?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain) | Coverage Δ | | |---|---|---| | [bluepysnap/edges/edges.py](https://app.codecov.io/gh/BlueBrain/snap/pull/239?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#diff-Ymx1ZXB5c25hcC9lZGdlcy9lZGdlcy5weQ==) | `100.00% <ø> (ø)` | | | [bluepysnap/nodes/nodes.py](https://app.codecov.io/gh/BlueBrain/snap/pull/239?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BlueBrain#diff-Ymx1ZXB5c25hcC9ub2Rlcy9ub2Rlcy5weQ==) | `100.00% <ø> (ø)` | |

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