manual: Unify EXAMPLE title in code and manual

There are 3 types of EXAMPLE title in manual and code:
1: "# EXAMPLE"
   runc-delete.8.md
   runc-exec.8.md
   runc-kill.8.md
2: "EXAMPE:"
   runc-spec.8.md
3: EXAMPLE title exist in manual, but not exist in code's --help output
   delete.go
   exec.go
   kill.go

This patch unified above format, and deleted some useless blanks.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>

man/runc-spec.8.md

@@ -14,7 +14,7 @@ parameter that is initially set to call the "sh" command when the container is
 started. Calling "sh" may work for an ubuntu container or busybox, but will not
 work for containers that do not include the "sh" program.
 
-EXAMPLE:
+# EXAMPLE
   To run docker's hello-world container one needs to set the args parameter
 in the spec to call hello. This can be done using the sed command or a text
 editor. The following commands create a bundle for hello-world, change the