Update doc string for check flag of fmt command

Co-authored-by: Brian Cain <bcain@hashicorp.com>

internal/cli/fmt.go

@@ -130,7 +130,7 @@ func (c *FmtCommand) Flags() *flag.Sets {
  Target: &c.flagCheck,
  Default: false,
  Usage: "Check if the input is formatted. Exit status will be 0 if " +
- "all input is properly formatted and non-zero otherwise.",
+ "all input is properly formatted and exit status 3 otherwise.",
  })
  })
 }