FIX: correct while-loop semantics and an its/it's typo

Two pre-existing prose defects on lines this PR already touches, both raised
in Copilot's review of #604 and listed as out-of-scope candidates in the PR
description.

* python_by_example.md: a while loop runs *as long as* its condition holds,
  not "until the condition is satisfied", which said the loop starts when the
  condition is false and stops when it becomes true. The next sentence already
  described the behaviour correctly, so the two contradicted each other.
* functions.md: "each successive call uses it's own frame" -> "its".

Prose only; no code cells change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

lectures/functions.md

@@ -447,7 +447,7 @@ def x(t):
         return 2 * x(t-1)
 ```
 
-What happens here is that each successive call uses it's own *frame* in the *stack*
+What happens here is that each successive call uses its own *frame* in the *stack*
 
 * a frame is where the local variables of a given function call are held
 * stack is memory used to process function calls

lectures/python_by_example.md

@@ -380,7 +380,7 @@ plt.plot(ϵ_values)
 plt.show()
 ```
 
-A while loop will keep executing the code block delimited by indentation until the condition (`i < ts_length`) is satisfied.
+A while loop will keep executing the code block delimited by indentation as long as the condition (`i < ts_length`) is satisfied.
 
 In this case, the program will keep adding values to the list `ϵ_values` until `i` equals `ts_length`: