Add a couple missing words in the config file docs (#97)

Author: autarch

source/reference/configuration.txt

@@ -20,7 +20,7 @@ file.  The configuration file contains settings that are the equivalent of
 Configuration File
 ==================
 
-Most :program:`mongosync` command-line options can be written to file.
+Most :program:`mongosync` command-line options can be written to a YAML file.
 The configuration file specifies values for each setting using YAML format.
 For example: