config: Use spaces (not tabs) to indent JSON

Change made with:

  $ sed -i 's/\t/    /g' config.md

fixing tabs that were added with 1c49f4d (Add annotations and labels
to the Spec, 2016-03-04, opencontainers#331).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

config.md

@@ -304,8 +304,8 @@ Annotations are key-value maps.
 
 ```json
 "annotations": {
-	"key1" : "value1",
-	"key2" : "value2"
+    "key1" : "value1",
+    "key2" : "value2"
 }
 ```