Fix typo in chart README

[janpieper]

Thank you for making node-red ⚙ better

Found a typo in the chart README :wink:

Also verify you have:

• [x] Read the contributions page.
• [x] Read the DCO, if you are a first time contributor.
• [x] Read the [code of conduct](Code of Conduct).

[dirien]

cc: @patrick246