Fix documentation: Change cancellableBackgroundTask to backgroundCancellableTask

[Copilot]

The documentation incorrectly referenced the background cancellable task computation expression as cancellableBackgroundTask when the correct name is backgroundCancellableTask.

This affected two documentation files:

• README.md - line 17 in the CancellableTask section
• docsSrc/index.md - line 16 in the CancellableTask section

The actual computation expression builder is defined in src/IcedTasks/CancellableTask.fs as:

let backgroundCancellableTask = BackgroundCancellableTaskBuilder()

This fix ensures the documentation matches the actual implementation and provides users with the correct computation expression name to use in their code.

Fixes issue where users following the documentation would get compilation errors when trying to use the incorrectly named cancellableBackgroundTask computation expression.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix documentation</issue_title>
<issue_description>Hello, I discovered something wrong in documentation. In docs, for background cancellable task, you need to use cancellableBackgroundTask, but the correct name is backgroundCancellableTask. Maybe there are other mispelled computation expressions, but didn't checked them :)</issue_description>

<agent_instructions>Focus solely on fixing the documentation in docsSrc or README. Do not change any source code. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes #56

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