Explain command for a specific file doesn't need codebase variable

Issue

The codebase variable doesn't add any value to the command in step 10, it is not required to search the codebase to explain a single file.

Where this is

Analyze code using prompts in the Chat view

Step 10:
Enter a prompt in the Chat view that generates an explanation of the Program.cs file.
/explain #codebase Explain the Program.cs file

What it should be

/explain #Program.cs

This command is enough and to the point and uses the least number of tokens.

打开“Analyze code using prompts in the Chat view”说明，并检查当前使用 /explain #codebase Explain the Program.cs file 的第 10 步。将其替换为请求的 /explain #Program.cs 命令，并确认该步骤仍清楚地说明了如何为 Program.cs 生成解释。