Change some typography in the docs.

Two issues are addressed with this commit:

1. Some `<pre>` tags were inheriting the "Serif" font on Linux, causing
"code"-ish stuff to appear with proportional-width font instead of
monospaced-width font.
2. Font stack with "Helvetica Neue" ... has been changed to default to
sans-serif instead of "Serif", this produces a more consistent and
friendlier look on Windows in particular.

Also, "Helvetica" was misspelled.