[Doc] use ${localWorkspaceFolderBasename} in named volume example

[czzonet]

In example: Improve container performance

"mounts": [
    "source=try-node-node_modules,target=${containerWorkspaceFolder}/node_modules,type=volume"
]

Better to use ${localWorkspaceFolderBasename} for readers to reuse instead of the static name.

"mounts": [
  "source=${localWorkspaceFolderBasename}-node_modules,target=${containerWorkspaceFolder}/node_modules,type=volume"
]

[chrmarti]

There is now also ${devcontainerId}.