MINOR: [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

[raulcd]

This looks good to me an in-line with our example in:

arrow/docs/source/cpp/parquet.rst

Lines 297 to 303 in 23cd1ff

	std::shared_ptr<WriterProperties> props = WriterProperties::Builder()
	.compression(Compression::SNAPPY) // Fallback
	->compression("colA", Compression::ZSTD) // Only applies to column "colA"
	->encoding(Encoding::BIT_PACKED) // Fallback
	->encoding("colB", Encoding::RLE) // Only applies to column "colB"
	->disable_dictionary("colB") // Never dictionary-encode column "colB"
	->build();

[alexeyroytman]

My further only concern was the indentation (2 in my case, 3 in the case @raulcd mentioned).