Correct path to age keys.txt in documentation (#861)

corrected path to keys based on the real behavior. see [1]

[1]
https://github.com/mozilla/sops/blob/master/age/keysource.go#L108

README.rst

@@ -193,9 +193,9 @@ the ``--age`` option or the **SOPS_AGE_RECIPIENTS** environment variable:
 
 When decrypting a file with the corresponding identity, sops will look for a
 text file name ``keys.txt`` located in a ``sops`` subdirectory of your user
-configuration directory. On Linux, this would be ``$XDG_CONFIG_HOME/sops/keys.txt``.
-On macOS, this would be ``$HOME/Library/Application Support/sops/keys.txt``. On
-Windows, this would be ``%AppData%\sops\keys.txt``. You can specify the location
+configuration directory. On Linux, this would be ``$XDG_CONFIG_HOME/sops/age/keys.txt``.
+On macOS, this would be ``$HOME/Library/Application Support/sops/age/keys.txt``. On
+Windows, this would be ``%AppData%\sops\age\keys.txt``. You can specify the location
 of this file manually by setting the environment variable **SOPS_AGE_KEY_FILE**.
 
 The contents of this key file should be a list of age X25519 identities, one