Fix JSON Mapping Example in Docs

[erjel]

extra {} cause JSON decode error in python standard JSON library and webknossos instance

Steps to test:

import json

"""
json.loads(
"""
{
  {
    "name": "astrocytes",
    "classes": [
      [
        69381,
        69445,
        138248
      ],
      [
        138307,
        343831
      ],
      [
        348348,
        132432,
        387433,
        338330
      ]
    ]
  }
}
""")

results in:

JSONDecodeError: Expecting property name enclosed in double quotes: line 3 column 3 (char 5)

---

(Please delete unneeded items, merge only when none are left open)

• Updated documentation if applicable
• Ready for review

[CLAassistant]

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[philippotto]

Thanks for fixing this! Once you've signed, the CLA I can merge this PR.