Missing DS for bootstrap file path in Console\Command\Task\PluginTask

to avoid warning during plugin generation via bake:

```
Warning Error: file_get_contents(/bootstrap.php): failed to open stream: No such file or directory in [apps/dev/vendor/cakephp/cakephp/src/Utility/File.php, line 149]

Warning: Warning (2): file_get_contents(/bootstrap.php): failed to open stream: No such file or directory in [apps/dev/vendor/cakephp/cakephp/src/Utility/File.php, line 149]
Trace:
Cake\Error\BaseErrorHandler::handleError() - CORE/src/Error/BaseErrorHandler.php, line 135
file_get_contents - [internal], line ??
Cake\Utility\File::read() - CORE/src/Utility/File.php, line 149
Cake\Console\Command\Task\PluginTask::_modifyBootstrap() - CORE/src/Console/Command/Task/PluginTask.php, line 165
Cake\Console\Command\Task\PluginTask::bake() - CORE/src/Console/Command/Task/PluginTask.php, line 144
Cake\Console\Command\Task\PluginTask::main() - CORE/src/Console/Command/Task/PluginTask.php, line 72
Cake\Console\Shell::runCommand() - CORE/src/Console/Shell.php, line 375
Cake\Console\Shell::runCommand() - CORE/src/Console/Shell.php, line 370
Cake\Console\ShellDispatcher::_dispatch() - CORE/src/Console/ShellDispatcher.php, line 163
Cake\Console\ShellDispatcher::dispatch() - CORE/src/Console/ShellDispatcher.php, line 139
Cake\Console\ShellDispatcher::run() - CORE/src/Console/ShellDispatcher.php, line 96
[main] - APP/Console/cake.php, line 20
```