flatbuffers.h: fix documentation warning (#5607)

Fixes following clang -Wdocumentation warning:
```
flatbuffers.h:1762:17: error: parameter ']' not found in the function declaration [-Werror,-Wdocumentation]
  /// @param[in]] v A const reference to the `std::vector` of structs to
```

include/flatbuffers/flatbuffers.h

@@ -1759,7 +1759,7 @@ class FlatBufferBuilder {
   /// `vector`.
   /// @tparam T The data type of the `std::vector` struct elements.
   /// @tparam S The data type of the `std::vector` native struct elements.
-  /// @param[in]] v A const reference to the `std::vector` of structs to
+  /// @param[in] v A const reference to the `std::vector` of structs to
   /// serialize into the buffer as a `vector`.
   /// @return Returns a typed `Offset` into the serialized data indicating
   /// where the vector is stored.