Improve httpx instrumentation examples

[andremm]

Description

Some instrumentation examples inside httpx don't run. This PR fixes the examples, so they run. It also ports the fixes to README.rst and makes indentation consistent across examples.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

1. Clone contrib repo: git clone https://github.com/open-telemetry/opentelemetry-python-contrib
2. Clone OTel Python repo: git clone https://github.com/open-telemetry/opentelemetry-python
3. Access the contrib repo: cd opentelemetry-python-contrib
4. Create a virtual env: python3 -m venv otelvenv
5. Activate the virtual env: source otelvenv/bin/activate
6. Install common dependencies:

pip install opentelemetry-distro/ opentelemetry-instrumentation/ \
 ../opentelemetry-python/opentelemetry-semantic-conventions/ \
 ../opentelemetry-python/opentelemetry-api/ \
 ../opentelemetry-python/opentelemetry-sdk/ \
 ../opentelemetry-python/opentelemetry-proto/ \
 ../opentelemetry-python/exporter/opentelemetry-exporter-otlp-proto-common

7. Install httpx specific requirements:

pip install -r instrumentation/opentelemetry-instrumentation-httpx/test-requirements.txt

8. Copy the instrumentation examples inside the instrumentation main __init__.py file into different Python files and try to run them.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated