Remove [--editable] in install instructions.

Use -e only in docs targeted at contributors.

If a user copy-pastes  command line with [--editable], they will hit
an error. If they don't know the --editable option, we're giving them
a choice to make before they can move forwards, but this isn't a choice
they need to make right now.

examples/summarization/README.md

@@ -10,7 +10,7 @@ The model is loaded with the pre-trained weights for the abstractive summarizati
 
 ```
 git clone https://github.com/huggingface/transformers && cd transformers
-pip install [--editable] .
+pip install .
 pip install nltk py-rouge
 cd examples/summarization
 ```