Minor Error in Cluster Documentation

[gordonbanderson]

In https://github.com/manticoresoftware/manticoresearch-php/blob/master/docs/cluster.md

The title for the documentation to delete a cluster is 'Create'

[githubmanticore]

fix #31 delete cluster docs via commit e249823

[githubmanticore]

fix #31 delete cluster docs via commit e249823