documentation: simplify pdf diagram

NOTRY=true
TBR=

Review URL: https://codereview.chromium.org/1810943003
diff --git a/site/dev/design/PdfLogicalDocumentStructure.png b/site/dev/design/PdfLogicalDocumentStructure.png
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+++ b/site/dev/design/PdfLogicalDocumentStructure.png
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