expire 0 -> null

XX.md

@@ -37,7 +37,7 @@ The entry must be structured as follows:
 "nuts": {
     ...,
     "xx": {
-        "ttl": <int>,
+        "ttl": <int|null>,
         "cached_endpoints": [
             {
                 "path": "v1/{mint|melt}/{method}",
@@ -56,7 +56,7 @@ The entry must be structured as follows:
 Where `ttl` is the amount of seconds the responses are cached for and `cached_endpoints` is a list of the routes for which caching is enabled.
 `path` and `method` describe respectively the cached route and its method.
 
-if `ttl` is set to 0, then the responses are to be considered cached *indefinetely*
+if `ttl` is null, the responses are to be considered cached *indefinetely*.
 
 ## Example