Update README.md: clarified the manual installation and corrected some typos

[MatthiasOs]

Hey, thanks for the project.
I just used the instructions from "Manual" to successfully integrate victron data into home assistant.

In this PR

Update README.md:

• clarified the manual installation
• corrected some typos

Breaking change

No Breaking changes.

Proposed change

Update Readme

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

No additional information.

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• The code has been formatted using Ruff (ruff format custom_components/victron)

[MatthiasOs]

It looks like i cant set the label myself, to pass the github check.
"documentation" would be the most fitting.

[sfstar]

LGTM, thank you for your contribution @MatthiasOs

[sfstar]

Merging with ruff failure ignoring due to PR only having documentation / readme changes.