Update index.MD to reflect new vue CLI 3.x.x

[Trellian]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• [x ] Provided documentation update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• [x ] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch and not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested with all major browsers

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
The old version is correct for installing the v2.x.x Vue CLI, but the page already asks to check for v3.x.x.
This change updates to the correct instructions for installing v3.x.x as per [https://cli.vuejs.org/guide/installation.html]