pages/*: add backticks around I/O streams (#10436)

* as suggested in #10428
* standard input  => `stdin`
* standard output => `stdout`
* standard error  => `stderr`

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/pdftotext.md

@@ -3,7 +3,7 @@
 > Convert PDF files to plain text format.
 > More information: <https://www.xpdfreader.com/pdftotext-man.html>.
 
-- Convert `filename.pdf` to plain text and print it to standard output:
+- Convert `filename.pdf` to plain text and print it to `stdout`:
 
 `pdftotext {{filename.pdf}} -`