[prompt] Update prompt.md (#1839)

## Summary
Additional clarifications to our prompt. Still very concise, but we'll
continue to add more here.

codex-rs/core/src/openai_tools.rs

@@ -55,7 +55,7 @@ fn create_shell_tool() -> OpenAiTool {
 
     OpenAiTool::Function(ResponsesApiTool {
         name: "shell",
-        description: "Runs a shell command, and returns its output.",
+        description: "Runs a shell command and returns its output",
         strict: false,
         parameters: JsonSchema::Object {
             properties,