docs: fix comment for LedgerHistory::fixIndex return value (#4574)

`LedgerHistory::fixIndex` returns `false` if a repair was performed.

Fix #4572

src/ripple/app/ledger/LedgerHistory.h

@@ -95,7 +95,7 @@ class LedgerHistory
     /** Repair a hash to index mapping
         @param ledgerIndex The index whose mapping is to be repaired
         @param ledgerHash The hash it is to be mapped to
-        @return `true` if the mapping was repaired
+        @return `false` if the mapping was repaired
     */
     bool
     fixIndex(LedgerIndex ledgerIndex, LedgerHash const& ledgerHash);