doc: imx: psb: fix footnotes

In reStructured text footnotes are referenced like [1]_.

Add missing underscores.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: NMarek Vasut <marex@denx.de>

doc/imx/misc/psb.rst

@@ -88,7 +88,7 @@ SIT format
 SIT is a 20 byte long structure containing of 5 32-bit words. Those encode
 bootloader B-copy area offset (called "firstSectorNumber"), magic value
 (called "tag") that is always 0x00112233, and three unused words set to 0.
-SIT is documented in [1] and [2]. Example SIT are below::
+SIT is documented in [1]_ and [2]_. Example SIT are below::
 
   $ hexdump -vC sit-mx7d.bin
     00000000  00 00 00 00