doc(readme): fix incorrect install command (invoke-ai#3024)

Hi, I am trying to install InvokeAI on my linux machine, the command in
README.md cannot install correct dependency

Author: lstein

README.md

@@ -139,7 +139,7 @@ not supported.
     _For Windows/Linux with an NVIDIA GPU:_
 
     ```terminal
-    pip install InvokeAI[xformers] --use-pep517 --extra-index-url https://download.pytorch.org/whl/cu117
+    pip install "InvokeAI[xformers]" --use-pep517 --extra-index-url https://download.pytorch.org/whl/cu117
     ```
 
     _For Linux with an AMD GPU:_