Revert "Bump rdoc from 6.2.0 to 6.2.1"

This reverts commit 4e26dfc.

Try to fix deploy failure
```
bundle stdout: rdoc-6.2.1 requires ruby version >= 2.4.0, which is incompatible with the
current version, ruby 2.3.3p222
bundle stderr: Nothing written
```

Gemfile.lock

@@ -19,7 +19,7 @@ GEM
       net-ssh (>= 2.6.5, < 6.0.0)
     net-ssh (5.2.0)
     rake (13.0.1)
-    rdoc (6.2.1)
+    rdoc (6.2.0)
     sshkit (1.20.0)
       net-scp (>= 1.1.2)
       net-ssh (>= 2.8.0)