Update plugins.rst for SFTP (#130)

* Update plugins.rst

Unlike what the current page says, the FTP server plugin does not provide SFTP (it provides FTP and FTPS), while there is another plugin for FTP. Having a single line misleads users interested in SFTP to install the FTP plugin and wonder how to get SFTP.

* Minor changes

Signed-off-by: Volker Theile <votdev@gmx.de>

---------

Signed-off-by: Volker Theile <votdev@gmx.de>
Co-authored-by: Volker Theile <votdev@gmx.de>
(cherry picked from commit db06976)
Signed-off-by: Volker Theile <votdev@gmx.de>

plugins.rst

@@ -26,7 +26,7 @@ The following is the list of official plugins by |omv|. Their availability depen
 * **ClamAV**: Provides Clam AntiVirus (ClamAV). It is a free software, cross-platform and open-source antivirus software toolkit able to detect many types of malicious software, including viruses
 * **Diskstats**: Complementary plugin to extend system statistics collection by adding I/O statistic graphs.
 * **Forked-daap**: Provides a daap protocol music server.
-* **FTP**: Provides a modular FTP/SFTP/FTPS server.
+* **FTP**: Provides a modular FTP/FTPS server.
 * **LVM2**: LVM managing. Create volume groups and logical partitions.
 * **NUT**: Controlling and configuring UPS. The driver support is based on NUT.
 * **Onedrive** (amd64, arm64, armhf, i386 only): Synchronizing a shared folder with Microsoft OneDrive cloud storage.