environment.yml uses conda channel 'default' instead of 'defaults'; trailing space in _toc.yml

Two small defects in the build configuration, both found while porting this repo's scaffold into the new lecture-python-programming.ml edition (QuantEcon/lecture-python-programming.ml#6). Neither breaks the build today; the first is a latent failure worth getting ahead of.

1. environment.yml names the conda channel default, not defaults

default is not a conda channel. Conda adds it as a literal channel name and then resolves the packages from defaults, which it still adds implicitly — and warns about doing so:

Adding channel 'default'
[command]/home/runner/miniconda3/condabin/conda config --add channels default
##[warning]The 'defaults' channel might have been added implicitly. If this is intentional, add 'defaults' to the 'channels' list. Otherwise, consider setting 'conda-remove-defaults' to 'true'.

So the environment resolves by accident. Conda is deprecating the implicit-defaults behaviour, and when it goes the build fails to resolve anything.

This affects three editions — this repo, lecture-python-programming.fr and lecture-python-programming.fa — all of which carry - default. lecture-python-programming.zh-cn already has - defaults and is unaffected, so the corrected form is already proven in the fleet.

The fix is one word in environment.yml.

2. lectures/_toc.yml line 39 has a trailing space

The entry reads - file: workspace rather than - file: workspace. This is harmless in practice — YAML strips trailing whitespace from plain scalars, so both jb build and any tooling reading the file see workspace — but it propagates verbatim into every translation edition, since action-translation copies _toc.yml from source as-is. It surfaced as a review comment on the downstream PR, where fixing it locally would only have created drift against this repo.

Worth correcting here so it stops being re-copied.

Mở environment.yml và lectures/_toc.yml, sau đó so sánh cách viết channel với dạng đã được sửa và đang được dùng trong bản zh-cn. Đổi default thành defaults và xóa khoảng trắng ở cuối sau workspace; chạy jb build để xác nhận rằng cấu hình vẫn được phân giải và build thành công, không có lỗi.