doc: Add missing apostrophes to http.OutgoingMessage docs

Author: juanarbol

doc/api/http.md

@@ -2424,15 +2424,15 @@ added: v0.3.6
 
 Emitted when the buffer of the message is free again.
 
-### Event: `finish`
+### Event: `'finish'`
 
 <!-- YAML
 added: v0.1.17
 -->
 
 Emitted when the transmission is finished successfully.
 
-### Event: `prefinish`
+### Event: `'prefinish'`
 
 <!-- YAML
 added: v0.11.6