Fix bad method links in docstrings (#13471)

# Description

Seems like I developed a bit of a bad habit of trying to link

```rust
/// [`.foo()`]
```

in docstrings, and this just doesn't work automatically; you have to do 

```rust
/// [`.foo()`](Self::foo)
```

if you want it to actually link. I think I found and replaced all of
these.

# User-Facing Changes

Just docs.

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

@@ -80,8 +80,8 @@ impl PluginGc {
     ///
     /// The reason the plugin tells the GC rather than just stopping itself via `source` is that
     /// it can't guarantee that the plugin currently pointed to by `source` is itself, but if the
-    /// GC is still running, it hasn't received [`.stop_tracking()`] yet, which means it should be
-    /// the right plugin.
+    /// GC is still running, it hasn't received [`.stop_tracking()`](Self::stop_tracking) yet, which
+    /// means it should be the right plugin.
     pub fn exited(&self) {
         let _ = self.sender.send(PluginGcMsg::Exited);
     }

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

@@ -26,7 +26,7 @@ impl PluginSource {
 
     /// Create a new fake source with a fake identity, for testing
     ///
-    /// Warning: [`.persistent()`] will always return an error.
+    /// Warning: [`.persistent()`](Self::persistent) will always return an error.
     pub fn new_fake(name: &str) -> PluginSource {
         PluginSource {
             identity: PluginIdentity::new_fake(name).into(),