(website): correct usage of topic create (#704)

This PR corrects the example usages of `$ fluvio topic create`.

The `$ fluvio topic create` subcommand takes in the the topic name as a positional argument; `--topic` is not valid. Please refer to the following screenshot for more information:

<img width="1077" alt="Screen Shot 2023-10-18 at 9 42 18 PM" src="https://github.com/infinyon/fluvio-website/assets/45523555/0905952e-ac79-4a26-8aca-dd9b198562c6">

content/docs/architecture/replica-assignment.md

@@ -209,7 +209,7 @@ The following command creates a topic from a **replica assignment file**:
 
 %copy first-line%
 ```bash
-$ fluvio topic create --topic custom-topic --replica-assignment ./my-assignment
+$ fluvio topic create custom-topic --replica-assignment ./my-assignment
 ```
 
 _Validate-only_ flag is available to verify a replica assignment file without applying any changes.
@@ -244,4 +244,4 @@ The **replica map** definition meet the following criteria:
 
 For additional information on how to check the result of a replica assignment file, checkout [Topics CLI].
 
-[Topics CLI]: {{< ref "/cli/commands/topic" >}}
+[Topics CLI]: {{< ref "/cli/commands/topic" >}}

content/docs/concepts/topics.md

@@ -20,7 +20,7 @@ Replicas have a leader and one or more followers and distributed across all avai
 For example, when provisioning a topic with **2** partitions and **3** replicas:
 
 ```bash
-$ fluvio topic create --topic topic-a --partitions 2 --replication 3
+$ fluvio topic create topic-a --partitions 2 --replication 3
 ```
 
 **Leaders** maintain the primary data set and **followers** store a copy of the data. Leader and follower replications are assigned to independent **SPUs**: