doc: Add "vento -u" to documentation

I was about to release v1.1.0 until I forgot this command was completely
undocumented, so here it is.

build.rs

@@ -56,6 +56,12 @@ fn vento() -> Result<Page> {
                 .long("--switch")
                 .help("Switches inventory slots"),
         )
+        .flag(
+            Flag::new()
+                .short("-u")
+                .long("--undo")
+                .help("Undoes the last action"),
+        )
         .flag(
             Flag::new()
                 .short("-i")

src/help.rs

@@ -29,6 +29,7 @@ pub fn vento() -> Result<()> {
 {}
     - {}: Lists files in selected inventory
     - {}: Switches slots
+    - {}: Undoes the last action
     - {}: Initializes Vento
     - {}: Displays this message",
         "Vento".bold().blue(),
@@ -37,6 +38,7 @@ pub fn vento() -> Result<()> {
             .bold()
             .green(),
         "vento ( -c | --switch )".bold().green(),
+        "vento ( -u | --undo )".bold().green(),
         "vento ( -i | --init )".bold().green(),
         "vento ( -h | --help )".bold().green()
     );