Update code example to use "is not" in Static Typing

[KleidonBD]

Godot 4.3 added the ability to use "is not" instead of "not a is b" for improved readability. This code example should be updated to use the new feature to be more up to date with the latest version.

Also, the original line used parentheses in its conditional statement, which is explicitly against the style guide.

[tetrapod00]

Duplicate of #9777 (although the older PR chooses to add another line to document is not rather than replacing the current one).

[tetrapod00]

This seems like a fine change.

Using the is not keyword without first documenting is is a bit strange. But this note block is meant to document type casting, not the is keyword itself, and there are two other examples on this page that use the is keyword:

godot-docs/tutorials/scripting/gdscript/static_typing.rst

Line 424 in 9a44730

``is`` keyword and then declare a variable with the type ``MyScript`` on

godot-docs/tutorials/scripting/gdscript/static_typing.rst

Line 466 in 9a44730

property contains an object of the right type using the ``is`` keyword, and

Between this PR and #9777, I slightly prefer the other PR. But this one is good and does not need additional changes.

[KleidonBD]

Completely understandable. Like you said, the keyword itself isn't the focus of the code block which is why I didn't feel the need to call attention to it. The main thing I sought to change was removing the parentheses to align with the GDScript style guide, and figured this would kill two birds with one stone.