Adjust PageMode constants to reflect PDF spec

I noticed that using the `/UseFullscreen` mode flag wasn't initiating full screen in Acrobat Reader, so I looked into the [Official PDF Reference Spec](https://wwwimages2.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf) (Page 74, Table 28) and found two mode constants were incorrect in PyPDF's implementation. The correct constants are:

| Value              | Description                                                                                                                   |
|--------------------|-------------------------------------------------------------------------------------------------------------------------------|
|**`UseNone`**       |Neither document outline nor thumbnail images visible                                                                          |
|**`UseOutlines`**   |Document outline visible                                                                                                       |
|**`UseThumbs`**     |Thumbnail images visible                                                                                                       |
|**`FullScreen`**    |Full-screen mode, with no menu bar, window controls, or any other window visible (_`UseFullscreen` in PyPDF2 is incorrect_)    |
|**`UseOC`**         |_(PDF 1.5)_ Optional content group panel visible                                                                               |
|**`UseAttachments`**|_(PDF 1.6)_ Attachments panel visible (_`UseAttach` in PyPDF2 is incorrect_)                                                   |