Fix ingest cross-doc links

This commit fixes some cross-doc links from the old ingest plugins page
to the new ingest processor pages that arose after converting
ingest-geoip and ingest-user-agent to modules.

docs/plugins/ingest-user-agent.asciidoc

@@ -3,5 +3,5 @@
 
 The `user_agent` processor is no longer distributed as a plugin, but is now a module
 distributed by default with Elasticsearch. See the
-{ref}/user-agent.html[User Agent processor] for more details.
+{ref}/user-agent-processor.html[User Agent processor] for more details.
 

docs/plugins/ingest.asciidoc

@@ -19,7 +19,7 @@ The `geoip` processor adds information about the geographical location of IP
 addresses, based on data from the Maxmind databases. This processor adds this
 information by default under the `geoip` field. The `geoip` processor is no
 longer distributed as a plugin, but is now a module distributed by default with
-Elasticsearch. See {ref}/ingest-geoip.html[Ingest `geoip` processor] for more
+Elasticsearch. See {ref}/geoip-processor.html[GeoIP processor] for more
 details.
 
 <<ingest-user-agent>>::
@@ -27,7 +27,7 @@ details.
 A processor that extracts details from the User-Agent header value. The
 `user_agent` processor is no longer distributed as a plugin, but is now a module
 distributed by default with Elasticsearch. See
-{ref}/ingest-user-agent.html[Ingest `user_agent` processor] for more details.
+{ref}/user-agent-processor.html[User Agent processor] for more details.
 
 [float]
 === Community contributed ingest plugins

docs/reference/ingest/processors/user-agent.asciidoc

@@ -1,4 +1,4 @@
-[[user-agent]]
+[[user-agent-processor]]
 === User Agent processor
 
 The `user_agent` processor extracts details from the user agent string a browser sends with its web requests.