fix missing argument in encrypting with age section (getsops#860)

Signed-off-by: Carsten Skov <carsten@simcax.dk>

README.rst

@@ -189,7 +189,7 @@ the ``--age`` option or the **SOPS_AGE_RECIPIENTS** environment variable:
 
 .. code:: bash
 
-   $ sops --age age1yt3tfqlfrwdwx0z0ynwplcr6qxcxfaqycuprpmy89nr83ltx74tqdpszlw test.yaml > test.enc.yaml
+   $ sops --encrypt --age age1yt3tfqlfrwdwx0z0ynwplcr6qxcxfaqycuprpmy89nr83ltx74tqdpszlw test.yaml > test.enc.yaml
 
 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