message: doc: change MathJax URL
diff: @@ -174,7 +174,7 @@ FORMULA_FONTSIZE = 14
  FORMULA_TRANSPARENT = YES
  USE_MATHJAX = YES
  MATHJAX_FORMAT = HTML-CSS
  -MATHJAX_RELPATH = http://cdn.mathjax.org/mathjax/latest
  +MATHJAX_RELPATH = http://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0
  MATHJAX_EXTENSIONS = TeX/AMSmath TeX/AMSsymbols
  MATHJAX_CODEFILE = @CMAKE_CURRENT_SOURCE_DIR@/mymath.js
  SEARCHENGINE = YES
  