docs(prerequisites): fix "Build Host" title

Add an empty line just before the "Build Host" title.

Without this, the title is not properly recognized, it does not get
added to the table of contents and the underlining characters appear
as dashes, as can be seen here:

https://trustedfirmware-a.readthedocs.io/en/v2.7/getting_started/prerequisites.html#prerequisites

Change-Id: Ia89cf3de0588495cbe64b0247dc860619f5ea6a8
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/docs/getting_started/prerequisites.rst b/docs/getting_started/prerequisites.rst
index 0b8a71c..179d17b 100644
--- a/docs/getting_started/prerequisites.rst
+++ b/docs/getting_started/prerequisites.rst
@@ -7,6 +7,7 @@
 It may possible to build |TF-A| with combinations of software packages that are
 different from those listed below, however only the software described in this
 document can be officially supported.
+
 Build Host
 ----------