serde: docs: escape html tags

Fixes the cargo docs warning:

warning: unclosed HTML tag `u8`
  --> proxmox-serde/src/lib.rs:55:18
   |
55 | /// Serialize Vec<u8> as base64 encoded string.
   |                  ^^^^
   |
   = note: `#[warn(rustdoc::invalid_html_tags)]` on by default
help: try marking as source code
   |
55 | /// Serialize `Vec<u8>` as base64 encoded string.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>

proxmox-serde/src/lib.rs

@@ -52,7 +52,7 @@ pub mod epoch_as_rfc3339 {
     }
 }
 
-/// Serialize Vec<u8> as base64 encoded string.
+/// Serialize `Vec<u8>` as base64 encoded string.
 ///
 /// Usage example:
 /// ```