doc: Misc syntax and spelling fixes

Tidying up a few Sphinx warnings that had built-up over time.
None of these are critical but it cleans up the Sphinx output.

At the same time, fixing some spelling errors that were detected.

Change-Id: I38209e235481eed287f8008c6de9dedd6b12ab2e
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/components/firmware-update.rst b/docs/components/firmware-update.rst
index 2bff00f..a591565 100644
--- a/docs/components/firmware-update.rst
+++ b/docs/components/firmware-update.rst
@@ -7,7 +7,7 @@
 This document describes the design of the Firmware Update (FWU) feature, which
 enables authenticated firmware to update firmware images from external
 interfaces such as USB, UART, SD-eMMC, NAND, NOR or Ethernet to SoC Non-Volatile
-memories such as NAND Flash, LPPDR2-NVM or any memory determined by the
+memories such as NAND Flash, LPDDR2-NVM or any memory determined by the
 platform. This feature functions even when the current firmware in the system
 is corrupt or missing; it therefore may be used as a recovery mode. It may also
 be complemented by other, higher level firmware update software.