doc: updated license in gemspec to 'apache-2.0'

Author: fransflippo

logstash-input-salesforce.gemspec

@@ -1,7 +1,7 @@
 Gem::Specification.new do |s|
   s.name = 'logstash-input-salesforce'
   s.version         = '3.3.0'
-  s.licenses = ['Apache License (2.0)']
+  s.licenses = ['apache-2.0']
   s.summary = "Creates events based on a Salesforce SOQL query"
   s.description     = "This gem is a Logstash plugin required to be installed on top of the Logstash core pipeline using $LS_HOME/bin/logstash-plugin install gemname. This gem is not a stand-alone program"
   s.authors = ["Russ Savage"]