docs: use single quotes to prevent interpolation

Fix the `Hello, World!` example in the README by using single quotes to
prevent shell interpolation of `!`.

Signed-off-by: Nick Hale <[email protected]>

Author: njhale

README.md

@@ -60,7 +60,7 @@ export OPENAI_API_KEY="your-api-key"
 ### 3. Run Hello World
 
 ```shell
-gptscript https://get.gptscript.ai/echo.gpt --input "Hello, World!"
+gptscript https://get.gptscript.ai/echo.gpt --input 'Hello, World!'
 ```
 
 ## How it works