docs: arm: fix a breakage with pdf output

Add an extra blank line, as otherwise XeLaTex will complain with:

! LaTeX Error: Too deeply nested.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/arm/spear/overview.rst

···
   Hierarchy in SPEAr is as follows:
 
   SPEAr (Platform)
+
 	- SPEAr3XX (3XX SOC series, based on ARM9)
 		- SPEAr300 (SOC)
 			- SPEAr300 Evaluation Board