Mention no underscore in named groups of grok patterns

[kmerz]

Context

Named groups in grok patterns can't be compiled when containing underscores. The java
pattern compiler do not allow them:
https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html#groupname

Expected Content

Named groups in patterns mustn't contain _ (e.g (?<field_name>\d.\d)) but should
rather look like: (?<fieldname>\d.\d)

Refs: Graylog2/graylog2-server#5563

[kmerz]

Not needed anymore. We fixed the issue instead.

Fixed by: Graylog2/graylog2-server#5704