Fix incorrect labels type

Author: juhlig

doc/prometheus_model_helpers.md

@@ -94,7 +94,7 @@ label_value() = term()
 
 
 <pre><code>
-labels() = [<a href="#type-labels">labels()</a>]
+labels() = [<a href="#type-labels">label()</a>]
 </code></pre>
 
 

src/model/prometheus_model_helpers.erl

@@ -49,7 +49,7 @@
 -type label_name() :: term().
 -type label_value() :: term().
 -type label() :: {label_name(), label_value()}.
--type labels() :: [labels()].
+-type labels() :: [label()].
 -type value() :: float() | integer() | undefined | infinity.
 -type prometheus_boolean() :: boolean() | number() | list() | undefined.
 -type gauge() :: value() | {value()} | {labels(), value()}.