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/pygmentize.md

@@ -3,7 +3,7 @@
 > Python-based syntax highlighter.
 > More information: <https://pygments.org/docs/cmdline/>.
 
-- Highlight file syntax and print to standard output (language is inferred from the file extension):
+- Highlight file syntax and print to `stdout` (language is inferred from the file extension):
 
 `pygmentize {{file.py}}`