Wrong grammar for mapping options in man page

The [man page](https://github.com/containers/fuse-overlayfs/blob/main/fuse-overlayfs.1.md#options) describes the syntax for UID mappings as `UID:MAPPED-UID:LEN[,UID2:MAPPED-UID2:LEN2]`, i.e., with `,` as mapping separator. However, the examples section makes it look like `:` needs to be used instead, and AFAIU the code `,` is wrong and `:` is correct. The same is true for the GID mapping.

Open fuse-overlayfs.1.md at the Options section and compare the mapping syntax with the examples for both UID and GID mappings. Check the implementation's option parser to confirm the accepted separator, then update the man-page wording and examples so they agree; done means both mappings are documented consistently.

Written by the indexing model from the issue text.