.python/SpellCheck: Add "RISCV" to "words" section

BZ:2562:
https://bugzilla.tianocore.org/show_bug.cgi?id=2562

Add RISCV to "words" section in cspell.base.yaml file to avoid spelling check error.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

.pytool/Plugin/SpellCheck/cspell.base.yaml

@@ -161,5 +161,6 @@
         "bootability",
         "Sdhci",
         "inmodule",
+        "RISCV",
     ]
 }