DOC: Fix clang warning unknown command tag name

Replace back-slash prefix for doxygen command with at-sign prefix.
This may eliminate the clang warning. at-sign seems to be the
preferred doxygen command prefix in VTK.

Change-Id: I2dbbff11a5c531f2dad0a231b7cb0bc1ed7ad66c
Merge topic 'ClangDocumentationWarning' into master

07e37872 DOC: Fix clang warning unknown command tag name