[C++][Parquet][Doc] pointer dereference instead of dot in parquet.rst

[alexeyroytman]

Rationale for this change

Example code did not compile because it used dot (.) instead of arrow (->) in C++ code.

What changes are included in this PR?

One C++ example code in a documentation to have dot (.) replaced by arrow (->).

Are these changes tested?

Manually, when I've tried to copy-and-paste example, and it did not compile.

Are there any user-facing changes?

One external example will change.

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview