Fix: add special character substitution to blocks with ampersands

While there was no problem rendering HTML (using `make`), rendering
docbook/XML failed, eg:

Error: Cannot parse the DocBook XML file:
xmlParseEntityRef: no name, line 814, column
40 (lttng-docs-2.13m39rfphx.xml, line 814)

Snippet of the failed section:

```
<screen role="term">$ cd $(mktemp -d) &&
  wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 &&
  tar -xf lttng-modules-latest-2.13.tar.bz2 &&
  cd lttng-modules-2.13.* &&
  make &&
  sudo make modules_install &&
  sudo depmod -a</screen>
```

The `tools/check.py` also fails without this modification.

Change-Id: I950fc010f05e8c3a3a1c8fecd963c23c62f19dc7
Signed-off-by: Kienan Stewart <kstewart@efficios.com>