[electrum] specify language for code blocks in markdown

Summary:
This enables syntax highlighting for code blocks in tools that support that feature.

Note that bash is an alias for shell (see https://github.com/github-linguist/linguist/blob/master/lib/linguist/languages.yml), and we already have some code blocks annotated with shell, so lets use it everywhere.

Test Plan: checked rendering in my IDE (PyCharm or CLion provide that)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14286