plugin.cfg: go gen -> go generate (#3511)

This takes over #3476 because that is a good change but lacks the DCO
and the author seems unresponsive.

Reformat the file a bit more while we're at it.

Closes: #3476
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin.cfg

-# Directives are registered in the order they should be
-# executed.
+# Directives are registered in the order they should be executed.
 #
-# Ordering is VERY important. Every plugin will
-# feel the effects of all other plugin below
-# (after) them during a request, but they must not
-# care what plugin above them are doing.
+# Ordering is VERY important. Every plugin will feel the effects of all other
+# plugin below (after) them during a request, but they must not care what plugin
+# above them are doing.
 
-# How to rebuild with updated plugin configurations:
-# Modify the list below and run `go gen && go build`
+# How to rebuild with updated plugin configurations: Modify the list below and
+# run `go generate && go build`
 
-# The parser takes the input format of
+# The parser takes the input format of:
+#
 #     <plugin-name>:<package-name>
 # Or
 #     <plugin-name>:<fully-qualified-package-name>
 #
 # External plugin example:
+#
 # log:github.com/coredns/coredns/plugin/log
 # Local plugin example:
 # log:log