doc: add missing argument to function signature

Specify that `outgoingMessage.removeHeader()` takes a `name` argument
whose type is string.

Author: lpinca

doc/api/http.md

@@ -2644,12 +2644,14 @@ it inherits from `Stream`.
 
 The User should not call this function directly.
 
-### `outgoingMessage.removeHeader()`
+### `outgoingMessage.removeHeader(name)`
 
 <!-- YAML
-added:  v0.4.0
+added: v0.4.0
 -->
 
+* `name` {string} Header name
+
 Removes a header that is queued for implicit sending.
 
 ```js