Fix documentation indentation (#313)

Author: Krapaince

lib/gen_stage.ex

@@ -921,18 +921,18 @@ defmodule GenStage do
 
   The producer can do one of these:
 
-    * Dispatch exactly as many events as `demand`.
-
-    * Dispatch *more events* than `demand` - in this case, GenStage will
-      buffer the excess events. These events will be "used" by the
-      consumer/dispatcher next time demand is sent upstream. It's only
-      once the events in the buffer don't satisfy the demand anymore that
-      the `c:handle_demand/2` callback is invoked again. See the "Buffering"
-      section in the module documentation.
-
-    * Dispatch less events than `demand` - in this case, the producer is
-      responsible for storing the demand ("buffering demand") and then emitting
-      events when they are available.
+  * Dispatch exactly as many events as `demand`.
+
+  * Dispatch *more events* than `demand` - in this case, GenStage will
+    buffer the excess events. These events will be "used" by the
+    consumer/dispatcher next time demand is sent upstream. It's only
+    once the events in the buffer don't satisfy the demand anymore that
+    the `c:handle_demand/2` callback is invoked again. See the "Buffering"
+    section in the module documentation.
+
+  * Dispatch less events than `demand` - in this case, the producer is
+    responsible for storing the demand ("buffering demand") and then emitting
+    events when they are available.
 
   See the "Demand" section in the module documentation.