[docs] Switch contraction to possesive

docs/reference/modules/discovery/zen.asciidoc

@@ -64,7 +64,7 @@ Nodes can be excluded from becoming a master by setting `node.master` to `false`
 
 The `discovery.zen.minimum_master_nodes` sets the minimum
 number of master eligible nodes that need to join a newly elected master in order for an election to
-complete and for the elected node to accept it's mastership. The same setting controls the minimum number of
+complete and for the elected node to accept its mastership. The same setting controls the minimum number of
 active master eligible nodes that should be a part of any active cluster. If this requirement is not met the
 active master node will step down and a new master election will be begin.