Doc: Use forward slashes for paths in the Linux deployment guide

The table of files needed to deploy the Plug & Paint example on Linux
wrote the plugin paths with Windows-style backslashes, such as
plugins\libpnp_basictools.so. Linux separates directories with forward
slashes, so the paths were wrong as written.

Use forward slashes instead.

Pick-to: 6.12
Change-Id: I2b32945bd469de79e2b3a666d3a0aa539dbb6697
Reviewed-by: Topi Reinio <topi.reinio@qt.io>