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/aspell.md

@@ -7,7 +7,7 @@
 
 `aspell check {{path/to/file}}`
 
-- List misspelled words from standard input:
+- List misspelled words from `stdin`:
 
 `cat {{path/to/file}} | aspell list`
 
@@ -19,6 +19,6 @@
 
 `aspell --lang={{cs}}`
 
-- List misspelled words from standard input and ignore words from personal word list:
+- List misspelled words from `stdin` and ignore words from personal word list:
 
 `cat {{path/to/file}} | aspell --personal={{personal-word-list.pws}} list`