docs: refactor example commands for clarity and consistency

- Remove `/topics/` prefix from the topic name in the example command
- Split the single notification command into multiple lines for clarity
- Replace `-k "API key"` with `--fcm-key "FCM Credentials Key Path"` in the example command

Signed-off-by: appleboy <appleboy.tw@gmail.com>

README.md

@@ -352,7 +352,7 @@ gorush -android -m "your message" -t "device token"
 Send messages to topics.
 
 ```bash
-gorush --android --topic "/topics/foo-bar" \
+gorush --android --topic "foo-bar" \
   -m "This is a Firebase Cloud Messaging Topic Message" \
   --fcm-key "FCM Credentials Key Path"
 ```
@@ -418,7 +418,9 @@ $ gorush -ios -m "your message" -i "your certificate path" \
 Send single notification with the following command:
 
 ```bash
-gorush -android -m "your message" -k "API key" -t "Device token"
+gorush -android -m "your message" \
+  --fcm-key "FCM Credentials Key Path" \
+  -t "Device token"
 ```
 
 ## Run gorush web server