have -timestamp-ui match what's in doc

this was just an omission
also `=true` is not a necessary precision in this case

#6784

command/build.go

@@ -30,7 +30,7 @@ func (c *BuildCommand) Run(args []string) int {
 	flags.BoolVar(&cfgColor, "color", true, "")
 	flags.BoolVar(&cfgDebug, "debug", false, "")
 	flags.BoolVar(&cfgForce, "force", false, "")
-	flags.BoolVar(&cfgTimestamp, "timestamp", false, "")
+	flags.BoolVar(&cfgTimestamp, "timestamp-ui", false, "")
 	flagOnError := enumflag.New(&cfgOnError, "cleanup", "abort", "ask")
 	flags.Var(flagOnError, "on-error", "")
 	flags.BoolVar(&cfgParallel, "parallel", true, "")
@@ -309,7 +309,7 @@ Options:
   -machine-readable             Machine-readable output
   -on-error=[cleanup|abort|ask] If the build fails do: clean up (default), abort, or ask
   -parallel=false               Disable parallelization (on by default)
-  -timestamp-ui=true            Prefix each ui output with an RFC3339 timestamp (off by default).
+  -timestamp-ui                 Enable prefixing of each ui output with an RFC3339 timestamp.
   -var 'key=value'              Variable for templates, can be used multiple times.
   -var-file=path                JSON file containing user variables.
 `
@@ -335,7 +335,7 @@ func (*BuildCommand) AutocompleteFlags() complete.Flags {
 		"-machine-readable": complete.PredictNothing,
 		"-on-error":         complete.PredictNothing,
 		"-parallel":         complete.PredictNothing,
-		"-timestamp":        complete.PredictNothing,
+		"-timestamp-ui":     complete.PredictNothing,
 		"-var":              complete.PredictNothing,
 		"-var-file":         complete.PredictNothing,
 	}

website/source/docs/commands/build.html.md

@@ -50,8 +50,8 @@ that are created will be outputted at the end of the build.
 -   `-parallel=false` - Disable parallelization of multiple builders (on by
     default).
 
--   `-timestamp-ui=true` - Prefix each ui output with an RFC3339 timestamp (off
-    by default).
+-   `-timestamp-ui` - Enable prefixing of each ui output with an RFC3339
+    timestamp.
 
 -   `-var` - Set a variable in your packer template. This option can be used
     multiple times. This is useful for setting version numbers for your build.