Fix duplicated words in documentation

[cupkk]

Fixes three duplicated words in user-facing documentation:

• can can in the bitsandbytes quantization guide
• from from in the scheduler timestep spacing guide
• better better in the Flux.2 caption upsampling guide

No code examples, APIs, or runtime behavior change.

Validation:

• asserted that each corrected sentence is present and each duplicated phrase is absent
• ran git diff --check
• verified the branch diff contains only the three documentation lines

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[github-actions]

Hi @cupkk, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.