DOC: fixed code output on copy_on_write.rst line 251

[ShaunSrirangam]

• closes DOC: Code example doesn't run as shown #63258 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
• If I used AI to develop this pull request, I prompted it to follow AGENTS.md.

Fixed the output of the code in docs:
df = pd.DataFrame({"foo": [1, 2, 3], "bar": [4, 5, 6]})
df["foo"][df["bar"] > 5] = 100
df

To:
foo bar
0 1 4
1 2 5
2 100 6

[mroeschke]

Thanks @ShaunSrirangam