add arial font to css code example

Example: font-family css

font-family: "Times New Roman", Georgia, serif;
/* If the browser does not support the first font (ie: Times New Roman),
it will then try the next font (ie: Georgia). The last font should always
be a "generic-family" font (ie: serif, sans-serif, etc..).
/* If font is more then one word, it must be put into quotes. */