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/linux/utmpdump.md

@@ -3,7 +3,7 @@
 > Dump and load btmp, utmp and wtmp accounting files.
 > More information: <https://manned.org/utmpdump>.
 
-- Dump the `/var/log/wtmp` file to the standard output as plain text:
+- Dump the `/var/log/wtmp` file to `stdout` as plain text:
 
 `utmpdump {{/var/log/wtmp}}`