Replaced dev core schema.xml with what is in hydra-head.

Currently, checkout the main branch of hydra jetty and running it in a jetty wrapper gives an error of: 

SolrCore Initialization Failures
development: org.apache.solr.common.SolrException:org.apache.solr.common.SolrException: Schema Parsing Failed: The content of elements must consist of well-formed character data or markup. 

To sanity check against it being just my machine, I had my colleague attempt a few checkout and run of it as well. In both of our cases, replacing the schema.xml with what is currently in hydra-head resolved the issue. If this is not the desired resolution, please feel free to ignore this pull request, but the current configuration isn't working for the development core.

Thanks.