Corrections in SSH section of dev-plugins documentation

- Remove the 'abstract' keyword for non-abstract methods
- Annotation 'desc' has been corrected to 'description'
- Add missing '@Override' annotations

Change-Id: Ic1b2d26d54fde337064f13188d88fc0d76ccfcb2

Documentation/dev-plugins.txt

@@ -437,9 +437,10 @@ Command implementations must extend the base class SshCommand:
 import com.google.gerrit.sshd.SshCommand;
 import com.google.gerrit.sshd.CommandMetaData;
 
-@CommandMetaData(name="print", descr="Print hello command")
+@CommandMetaData(name="print", description="Print hello command")
 class PrintHello extends SshCommand {
-  protected abstract void run() {
+  @Override
+  protected void run() {
     stdout.print("Hello\n");
   }
 }
@@ -455,7 +456,8 @@ import com.google.gerrit.sshd.SshCommand;
 
 @Export("print")
 class PrintHello extends SshCommand {
-  protected abstract void run() {
+  @Override
+  protected void run() {
     stdout.print("Hello\n");
   }
 }
@@ -470,6 +472,7 @@ with the `Gerrit-SshModule` attribute:
 import com.google.gerrit.sshd.PluginCommandModule;
 
 class MyCommands extends PluginCommandModule {
+  @Override
   protected void configureCommands() {
     command(PrintHello.class);
   }