docs: use the --dump form for the obsolete --dump-* flags

The SFTP page said `--dump-auth`, which no longer exists: it became a
value of `--dump`, so the docs asked for a flag rclone would reject.

The same line, and a line in the Swift troubleshooting section, also
used `--dump-headers` and `--dump-bodies`. Those still parse, but
SetFlags logs "--dump-headers is obsolete - please use --dump headers
instead", so the docs were steering readers onto a deprecated form.

The generated flag listings in docs/content/flags.md and
docs/content/commands/rclone.md are left alone: those flags do still
exist, so `--help` output should keep showing them.

docs/content/sftp.md

@@ -1442,7 +1442,7 @@ SFTP isn't supported under plan9 until [this
 issue](https://github.com/pkg/sftp/issues/156) is fixed.
 
 Note that since SFTP isn't HTTP based the following flags don't work
-with it: `--dump-headers`, `--dump-bodies`, `--dump-auth`.
+with it: `--dump headers`, `--dump bodies`, `--dump auth`.
 
 Note that `--timeout` and `--contimeout` are both supported.
 

docs/content/swift.md

@@ -709,7 +709,7 @@ Request" error rather than a more sensible error when the
 authentication fails for Swift.
 
 So this most likely means your username / password is wrong.  You can
-investigate further with the `--dump-bodies` flag.
+investigate further with the `--dump bodies` flag.
 
 This may also be caused by specifying the region when you shouldn't
 have (e.g. OVH).