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/unexpand.md

@@ -3,11 +3,11 @@
 > Convert spaces to tabs.
 > More information: <https://www.gnu.org/software/coreutils/unexpand>.
 
-- Convert blanks in each file to tabs, writing to standard output:
+- Convert blanks in each file to tabs, writing to `stdout`:
 
 `unexpand {{path/to/file}}`
 
-- Convert blanks to tabs, reading from standard output:
+- Convert blanks to tabs, reading from `stdout`:
 
 `unexpand`