Make tcpdump, wall and useradd match formatting.

While writing a parser I found that some files does not conform to the formatting rules. Changed the ones I found.
As a sidenote, I noticed that it currently does not show correct output from tcpdump (just installed via npm install -g tldr)

```
~ (master*)$ tldr tcpdump

  Dump traffic on a network

  - capture the traffic of a specific interface

    tcpdump -i eth0

  - capture all TCP traffic showing contents (ASCII) in console

    tcpdump -A tcp

  - capture the traffic from or to a host

    tcpdump host www.example.com

  - capture the traffic from a specific interface, source, destination and port

    tcpdump -i eth0 src 192.168.1.1 dest 192.168.1.2 and port 80

  - capture the traffic of a network

    tcpdump net 192.168.1.0/24

  - capture all traffic except traffic over port 22 and save to a dump file with today's date


```