doc: fix broken doc links (#13644)

Some broken doc links I saw when compiling with `cargo +stable doc
--no-deps --document-private-items --workspace --open`

crates/nu-plugin-engine/src/process.rs

@@ -79,7 +79,7 @@ impl PluginProcess {
         }
     }
 
-    /// Move the plugin process out of the foreground. See [`ForegroundGuard::reset`].
+    /// Move the plugin process out of the foreground. See [`ForegroundGuard`].
     ///
     /// This is a no-op if the plugin process was already in the background.
     pub(crate) fn exit_foreground(&self) -> Result<(), ShellError> {

crates/nu-plugin-engine/src/source.rs

@@ -11,7 +11,7 @@ pub struct PluginSource {
     /// A weak reference to the persistent plugin that might hold an interface to the plugin.
     ///
     /// This is weak to avoid cyclic references, but it does mean we might fail to upgrade if
-    /// the engine state lost the [`PersistentPlugin`] at some point.
+    /// the engine state lost the [`PersistentPlugin`][crate::PersistentPlugin] at some point.
     pub(crate) persistent: Weak<dyn GetPlugin>,
 }
 

crates/nu-plugin-engine/src/util/mutable_cow.rs

@@ -1,5 +1,5 @@
-/// Like [`Cow`] but with a mutable reference instead. So not exactly clone-on-write, but can be
-/// made owned.
+/// Like [`Cow`][std::borrow::Cow] but with a mutable reference instead. So not exactly
+/// clone-on-write, but can be made owned.
 pub enum MutableCow<'a, T> {
     Borrowed(&'a mut T),
     Owned(T),