docs: Remove redundant spec version from struct field access section

Struct field access is an enhancement to existing declarative calls feature,
not a separate spec version. The parent section already indicates availability
from spec version 1.2.0.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: fubhy

docs/subgraph-manifest.md

@@ -132,8 +132,6 @@ The `Expr` can be one of the following:
 
 #### Struct Field Access
 
-_Available from spec version 1.2.0_
-
 When event parameters contain struct types (tuples in ABI), you can access individual fields using either numeric indices or field names:
 
 **Numeric Access (Traditional):**