README: use backticks for code and commands (only)

This is a bit subjective and doesn't seem to have universal best
practices, but since the syntax is called "Code spans" according to the
CommonMark specification, it makes sense to only use them for (partial)
code or commands.

Signed-off-by: Michael Zimmermann <michael.zimmermann@sevenlab.de>

Author: M1cha

README.md

@@ -5,15 +5,15 @@
 The comments and instructions below are for the new IP stack in Zephyr.
 
 In a Zephyr default setup, the network tools are pre-installed under the
-`tools` directory.
+tools directory.
 
 Here are instructions for how to communicate between Zephyr running
 inside QEMU, and a host device that is running Linux.
 
 For setting up routing and NAT rules to allow access to external networks, please see
 [README NAT.md](README%20NAT.md)
 
-You need to run *socat* and *tunslip* to create a minimally working
+You need to run `socat` and `tunslip` to create a minimally working
 network setup.
 
 There are convenience scripts (_loop-socat.sh_ and _loop-slip-tap.sh_) for
@@ -101,8 +101,8 @@ $ net-setup.sh --config my-own-config.conf --iface foobar
 
 It is also possible to let the script return and then stop the network
 interface later. This can be done by first creating the interface with the
-"start" or "up" command, and then later remove the interface with the
-"stop" or "down" command.
+`start` or `up` command, and then later remove the interface with the
+`stop` or `down` command.
 ```
 $ net-setup.sh start
 do your things here
@@ -115,15 +115,14 @@ $ net-setup.sh --config my-own-config.conf down
 ```
 
 Any extra parameters that the script does not know, are passed directly
-to the "ip" command.
+to the `ip` command.
 ```
 $ net-setup.sh --config my-own-config.conf --iface foo user bar
 ```
 
 ## Using encrypted SSL link with echo-* programs
 
-Install stunnel
-
+Install `stunnel`.
 Fedora:
 ```
 $ dnf install stunnel
@@ -244,7 +243,7 @@ CONFIG_MBEDTLS_HEAP_SIZE=30000
 ## PPP Connectivity
 
 You can test the PPP connectivity running Zephyr in QEMU using a pppd that is
-running on the Linux host. You need to run *socat* and *pppd* to create
+running on the Linux host. You need to run `socat` and `pppd` to create
 a minimally working network setup.
 
 There are convenience scripts (_loop-ppp-dev.sh_ and _loop-pppd.sh_) for