Add missing quotes around value of snippets attribute
Closes
@@ -33,7 +33,7 @@ for the index operation:
[source,indent=0]
----
-operation::index[snippets=curl-request,http-request,http-response]
+operation::index[snippets='curl-request,http-request,http-response']
----
To include all of an operation's snippets, the `snippets` attribute can be omitted:
