Refactor: improve consistency of the term "directory".

This commit changes the term "folder" to "directory" in every
instance where the first is used in an example description, but the
second is used in the example code, and vice versa.

pages/common/pdfunite.md

@@ -6,6 +6,6 @@
 
 `pdfunite {{path/to/fileA.pdf}} {{path/to/fileB.pdf}} {{path/to/merged_output.pdf}}`
 
-- Merge a folder of PDFs into a single PDF:
+- Merge a directory of PDFs into a single PDF:
 
 `pdfunite {{path/to/directory/*.pdf}} {{path/to/merged_output.pdf}}`