[C++] Fix doxygen documentation of array.h

[asfimport]

this is just a small fix in doxygen documentation of the c++ api

Reporter: Viktor Gal / @vigsterkr
Assignee: Viktor Gal / @vigsterkr

PRs and other links:

• GitHub Pull Request #1442
• GitHub Pull Request #1442

_{Note: This issue was originally created as ARROW-1945. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 1442
#1442