Minor change to examples in Doxygen comments

Tags: #<Tag:0x00007fb37d4f28c8>

The C++ documentation style guide, as published a month ago, had a minor mistake: it recommended that examples be delimited by @code Doxygen tags. This ran counter to the long-standing (> 2 years) rule that Doxygen comments should use Markdown for formatting.

The guide has now been corrected, and requests that code blocks be delimited by four extra spaces compared to the surrounding text.