Fix IndentationError in worked example and stratify reference in Lab 8.2

[Yassir-249]

Fixes two bugs plus typos in course_8/gdm_lab_8_2_classification_part_1.ipynb.

Activity 4 worked example raises IndentationError when copied verbatim. In the "Worked example: Format the data" cell, the to_instruction_record docstring is indented 3 spaces while the function body is indented 4. Copying the template into the implementation cell (as the activity instructs) fails with IndentationError: unexpected indent. Re-indenting the docstring to 4 spaces lets the cell run and write the three JSONL splits.

Activity 3 task box stratifies on the wrong DataFrame. The alternative stratified-split snippet passes stratify=df['category'] when splitting df_val_test, which raises ValueError (inconsistent sample counts). Changed to stratify=df_val_test['category'], matching the worked example below it.

Typos: Disccover, excuted, funcion, processs, captstone, and a stray leading space on a code fence.

Ran the worked-example pipeline end-to-end to confirm the fixes.

[Yassir-249]

Rebased onto main. Both issues still reproduce on current main: the Activity 4 worked-example docstring is indented one space deeper than the function body (IndentationError), and the Activity 3 task-box second train_test_split stratifies by df while splitting df_val_test (ValueError). The diff is now just those two lines.