[pandas_panel] Describe the single .replace() call, not a loop (#645)

The pandas 3.0 update replaced the loop over continent names with one
.replace() call taking the whole list, but the sentence above it still
said we "loop through a list". Say what the code does, and end the
sentence with a full stop.

Intermediate publishes this lecture too; its copy takes the same change
in QuantEcon/lecture-python.myst#1088, so the two stay byte-identical.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: mmcky

lectures/pandas_panel.md

@@ -305,7 +305,7 @@ merged[merged['Country'] == 'Korea']
 
 We will also combine the Americas into a single continent - this will make our visualization nicer later on.
 
-To do this, we will use `.replace()` and loop through a list of the continent values we want to replace
+To do this, we will use `.replace()` with a list of the continent values we want to replace.
 
 ```{code-cell} ipython3
 replace = ['Central America', 'North America', 'South America']