docs: Remove additionalCategories from required fields (#543)

**What problem does this PR solve?**:
The other fields are required; we should point out that
additionalCategories is optional.

Thanks @deepakm-ntnx for spotting this!

**Which issue(s) this PR fixes**:
Fixes #

**How Has This Been Tested?**:
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Author: dlipovetsky

docs/content/customization/nutanix/machine-details.md

@@ -50,9 +50,6 @@ spec:
               subnets:
               - name: subnet-name
                 type: name
-              additionalCategories:
-              - key: example-key
-                value: example-value
               systemDiskSize: 40Gi
               vcpuSockets: 2
               vcpusPerSocket: 1
@@ -82,9 +79,6 @@ spec:
       subnet:
       - name: subnet-name
         type: name
-      additionalCategories:
-      - key: example-key
-        value: example-value
       systemDiskSize: 40Gi
       vcpuSockets: 2
       vcpusPerSocket: 1
@@ -112,9 +106,6 @@ spec:
       subnet:
       - name: subnet-name
         type: name
-      additionalCategories:
-      - key: example-key
-        value: example-value
       systemDiskSize: 40Gi
       vcpuSockets: 2
       vcpusPerSocket: 1