Fix network traffic http-request request_header

From the 2.1 spec:
> The corresponding value for each dictionary key MUST always be a list
of type string to support when a header field is repeated.

With thanks to @JasperJuergensen for spotting the issue and suggesting
the fix.

schemas/observables/network-traffic.json

@@ -198,7 +198,10 @@
               "description": "Specifies all of the HTTP header fields that may be found in the HTTP client request, as a dictionary.",
               "patternProperties": {
                 "^.+$": {
-                  "type": "string"
+                  "type": "array",
+                  "items": {
+                    "type": "string"
+                  }
                 }
               },
               "additionalProperties": false