*: always backquote stdin, stdout and stderr (#9584)

pages/common/envsubst.md

@@ -4,11 +4,11 @@
 > Variables to be replaced should be in either `${var}` or `$var` format.
 > More information: <https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html>.
 
-- Replace environment variables in stdin and output to stdout:
+- Replace environment variables in `stdin` and output to `stdout`:
 
 `echo '{{$HOME}}' | envsubst`
 
-- Replace environment variables in an input file and output to stdout:
+- Replace environment variables in an input file and output to `stdout`:
 
 `envsubst < {{path/to/input_file}}`