Manual pages: Standardize on EXAMPLE as section title

Manual pages: Standardize on EXAMPLE as section title

There is quite some value (in terms of readability and user
expectations) if consistent names are used for the sections
within manual pages. This patch is one of a series to bring
about this consistency.

Currently we have EXAMPLE (10) or EXAMPLES (23).
Let's standardize on the EXAMPLE (which is also what is
suggested in man-pages(7)) and used consistently across
a large number of pages in the Linux man-pages project.

(I realize the choice to go EXAMPLE, rather than EXAMPLES,
may be debatable. If necessary, I'd write a patch that instead
goes the other way, but I'd prefer to follow man-pages(7).)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>