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/stdbuf.md

@@ -3,14 +3,14 @@
 > Run a command with modified buffering operations for its standard streams.
 > More information: <https://www.gnu.org/software/coreutils/stdbuf>.
 
-- Change the standard input buffer size to 512 KiB:
+- Change `stdin` buffer size to 512 KiB:
 
 `stdbuf --input={{512K}} {{command}}`
 
-- Change the standard output buffer to line-buffered:
+- Change `stdout` buffer to line-buffered:
 
 `stdbuf --output={{L}} {{command}}`
 
-- Change the standard error buffer to unbuffered:
+- Change `stderr` buffer to unbuffered:
 
 `stdbuf --error={{0}} {{command}}`