Wrap word with __ with backticks for md formatting

The `__` is used for bold, and before this change was bolding and
overwrite format on remaining document.

Change-Id: Ia6c329013c60b2fc1c850e7b6dfaa0427a96426d

docs/kelvin_isa.md

diff --git a/docs/kelvin_isa.md b/docs/kelvin_isa.md
index 2c9cb28..50d9d27 100644
--- a/docs/kelvin_isa.md
+++ b/docs/kelvin_isa.md
@@ -875,7 +875,7 @@
   vd[L] = vs1[L] & MSB ? CLZ(~vs1[L]) : CLZ(vs1[L])
 ```
 
-Note: (clb - 1) is equivalent to __builtin_clrsb.
+Note: (clb - 1) is equivalent to `__builtin_clrsb`.
 
 **clb examples**