Some cleanup, fixes typos and grammar errors

core/agents/README.md

@@ -1,6 +1,6 @@
 This folder abstracts metadata lookup into "agents". Each agent can be implemented to get as
 much info as the external source provides, by using a granular set of interfaces 
-(see [interfaces](interfaces.go)].
+(see [interfaces](interfaces.go)).
 
 A new agent must comply with these simple implementation rules:
 1) Implement the `AgentName()` method. It just returns the name of the agent for logging purposes.
@@ -9,4 +9,4 @@ A new agent must comply with these simple implementation rules:
 
 For an agent to be used it needs to be listed in the `Agents` config option (default is `"lastfm,spotify"`). The order dictates the priority of the agents
 
-For a simple Agent example, look at the [placeholders.go](placeholders.go) agent source code.
+For a simple Agent example, look at the [placeholders](placeholders.go) agent source code.

core/artwork.go

@@ -113,7 +113,7 @@ func (a *artwork) getImagePath(ctx context.Context, id string) (path string, las
 		return
 	}
 
-	// If it is a mediaFile and it has cover art, return it (if feature is disabled, skip)
+	// If it is a mediaFile, and it has cover art, return it (if feature is disabled, skip)
 	if !conf.Server.DevFastAccessCoverArt && mf.HasCoverArt {
 		return mf.Path, mf.UpdatedAt, nil
 	}