Fix install instructions to use correct name (#166)

The actual package name (on both PyPI and conda-forge) is `jupyter-collaboration` with a `-`, not a `_`.

Author: fperez

docs/source/index.md

@@ -16,13 +16,13 @@ Editors are not collaborative by default; to activate it, install the extension
 Installation using mamba/conda:
 
 ```sh
-mamba install -c conda-forge jupyter_collaboration
+mamba install -c conda-forge jupyter-collaboration
 ```
 
 Installation using pip:
 
 ```sh
-pip install jupyter_collaboration
+pip install jupyter-collaboration
 ```
 
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