doc: Correct arch spellings in README.rst

Correct a couple of architecture capitalizations and spellings.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

README.rst

@@ -22,7 +22,7 @@ resource-constrained systems: from simple embedded environmental sensors and
 LED wearables to sophisticated smart watches and IoT wireless gateways.
 
 The Zephyr kernel supports multiple architectures, including ARM Cortex-M,
-Intel x86, ARC, NIOS II, Tensilica Xtensa, and RISC V, and a large number of
+Intel x86, ARC, Nios II, Tensilica Xtensa, and RISC-V, and a large number of
 `supported boards`_.
 
 .. below included in doc/introduction/introduction.rst