docs: Enhance token creation instructions with expiration advice (#100)

Author: ericr6

src/get-started/helloWorld/3-buildIApp.md

@@ -324,7 +324,7 @@ Deploy your iApp on the iExec protocol.
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-    <span>Name it "Test iExec iApp CLI" (expiration date is optional)</span>
+    <span>Name it "Test iExec iApp CLI". We highly recommend specifying an expiration date, typically a few days, weeks, or months in advance</span>
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     <span class="bg-gray-950 text-sm text-white w-6 h-6 rounded-full flex items-center justify-center font-medium">4</span>