Fix missing space in prompt for back and grep

**Problem description:**

This is a redux of #2161 without being quite as ambitious. See https://github.com/rapid7/metasploit-framework/pull/2161#issuecomment-21600253 for an example of the problem. This has been bugging me for a while.

**Verification steps:**

- [ ] ```use``` a module
- [ ] Run ```back```
- [ ] See that the prompt is correct
- [ ] Run ```grep grep grep```
- [ ] See that the prompt is correct

**Sample run:**

```
msf > use exploit/windows/smb/ms08_067_netapi
msf exploit(ms08_067_netapi) > back
msf > grep grep grep
Usage: grep [options] pattern cmd
Grep the results of a console command (similar to Linux grep command)
msf >
```