fix(hledger-fmt): add --exit-zero-on-changes (#859)

Author: piero-vic

lua/conform/formatters/hledger-fmt.lua

@@ -5,6 +5,6 @@ return {
     description = "An opinionated hledger's journal files formatter.",
   },
   command = "hledger-fmt",
-  args = { "--no-diff", "-" },
+  args = { "--no-diff", "--exit-zero-on-changes", "-" },
   stdin = true,
 }