docs: fix synatax error in Mangling.rst

Author: eeckstein

docs/ABI/Mangling.rst

@@ -717,7 +717,7 @@ conformance within the witness table, identified by the dependent type and
 protocol. In all cases, the DEPENDENT-CONFORMANCE-INDEX is an INDEX value
 indicating the position of the appropriate value within the generic environment
 (for "HD") or witness table (for "HI" and "HA") when it is known to be at a
-fixed position. An index of 1 ("0_") is used to indicate "unknown"; all other
+fixed position. An index of 1 ("0\_") is used to indicate "unknown"; all other
 values are adjusted by 2. That these indexes are not 0-based is a bug that's
 now codified into the ABI; the index 0 is therefore reserved.