Correct the code example for adding alternative inputs

Author: whyscream

ALTERNATIVE-INPUTS.md

@@ -7,9 +7,11 @@ Various other Logstash filters can produce the needed input fields (`program` an
 ----------------
 
 ``` 
-grok {
-    match => { "message" => "%{SYSLOGTIMESTAMP:timestamp} %{SYSLOGHOST} %{DATA:program}(?:\[%{POSINT}\])?: %{GREEDYDATA:message}" }
-    overwrite => ["timestamp", "message"]
+filter {
+    grok {
+        match => { "message" => "%{SYSLOGTIMESTAMP:timestamp} %{SYSLOGHOST} %{DATA:program}(?:\[%{POSINT}\])?: %{GREEDYDATA:message}" }
+        overwrite => ["timestamp", "message"]
+    }
 }
 ```