bugfix: 'headingRelevance' can't be Char. Changing to String.

Spark is not happy with it:

Exception in thread "main" scala.MatchError: scala.Char (of class scala.reflect.internal.Types$TypeRef$$anon$6)
        at org.apache.spark.sql.catalyst.ScalaReflection$.schemaFor(ScalaReflection.scala:53)
        at org.apache.spark.sql.catalyst.ScalaReflection$$anonfun$schemaFor$1.apply(ScalaReflection.scala:64)
        at org.apache.spark.sql.catalyst.ScalaReflection$$anonfun$schemaFor$1.apply(ScalaReflection.scala:62)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
        at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
        at scala.collection.immutable.List.foreach(List.scala:318)
        at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
        at scala.collection.AbstractTraversable.map(Traversable.scala:105)
        at org.apache.spark.sql.catalyst.ScalaReflection$.schemaFor(ScalaReflection.scala:62)
        at org.apache.spark.sql.catalyst.ScalaReflection$.schemaFor(ScalaReflection.scala:50)
        at org.apache.spark.sql.catalyst.ScalaReflection$.attributesFor(ScalaReflection.scala:44)
        at org.apache.spark.sql.execution.ExistingRdd$.fromProductRdd(basicOperators.scala:229)
        at org.apache.spark.sql.SQLContext.createSchemaRDD(SQLContext.scala:94)
        at canpipe.parser.spark.RunParser$.main(RunParser.scala:153)
        at canpipe.parser.spark.RunParser.main(RunParser.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.apache.spark.deploy.SparkSubmit$.launch(SparkSubmit.scala:331)
        at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:75)
        at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)