[python_by_example] While-loop description inverts the loop condition #599

In the "While loops" section, python_by_example.md currently reads:

A while loop will keep executing the code block delimited by indentation until the condition (i < ts_length) is satisfied.

The logic is inverted: a while loop executes the block while the condition is true, and stops once it fails. As written, "until the condition is satisfied" describes a loop that runs while i < ts_length is false.

Suggested wording:

A while loop will keep executing the code block delimited by indentation as long as the condition (i < ts_length) is true.

This matches the sentence that follows ("the program will keep adding values to the list until i equals ts_length"), which is stated correctly.

Found during native-editor review of the French edition — the machine translation had faithfully reproduced the inverted sentence, and the editor corrected it target-side in QuantEcon/lecture-python-programming.fr#25. Until it is corrected here, the French edition deliberately diverges from the source on this sentence. Same discovery path as #594.