change "LogStash" to "Logstash" in docs

"Logstash" is used consistently on the ES website.  This change
makes the documentation match up.

Did not change it in the book title "The LogStash Book" since
James uses mixed case for the book title at logstashbook.com.
"The LogStash Book" --> "The Logstash Book" per @jamtur01
Merge pull request #908 from bfritz/logstash-capitalization

change "LogStash" to "Logstash" in docs