Description

This PR fixes naming issue in battery_plus_linux. It's pubspec.yaml states that the plugin's class name is BatteryLinux, however throughout the code it was BatteryPlusLinux. For consistency with Linux implementations of other plugins, that do not use Plus in the name, I've removed that as well (instead of just renaming plugin's class name
in pubspec.yaml).

Without this building battery_plus_example on Mac fails with:

.dart_tool/flutter_build/generated_main.dart:19:9: Error: Getter not found: 'BatteryLinux'.
        BatteryLinux.registerWith();
        ^^^^^^^^^^^^

Command PhaseScriptExecution failed with a nonzero exit code
note: Using new build system
note: Planning
note: Build preparation complete
note: Building targets in parallel

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.