nifi.properties and admin guide refer to kerberos "principle" instead of "principal"

Need to change "principle" to "principal" in the following places:

In nifi.properties:

kerberos service principle
kerberos spnego principle

In Admin Guide for the description of the nifi.kerberos.krb5.file property:

At this time, only a single krb5 file is allowed to be specified per NiFi instance, so this property is configured here to support SPNEGO and service principles rather than in individual Processors.