Use concurrency in the README (#3428)

README.rst

@@ -22,17 +22,19 @@ Parsl lets you chain functions together and will launch each function as inputs
         return x + 1
 
     @python_app
-    def g(x):
-        return x * 2
+    def g(x, y):
+        return x + y
 
     # Start Parsl on a single computer
     with parsl.load():
-        # These functions now return Futures, and can be chained
+        # These functions now return Futures
         future = f(1)
         assert future.result() == 2
 
-        future = g(f(1))
-        assert future.result() == 4
+        # Functions run concurrently, can be chained
+        f_a, f_b = f(2), f(3)
+        future = g(f_a, f_b)
+        assert future.result() == 7
 
 
 Start with the `configuration quickstart <https://parsl.readthedocs.io/en/stable/quickstart.html#getting-started>`_ to learn how to tell Parsl how to use your computing resource,

docs/index.rst

@@ -30,8 +30,10 @@ Parsl lets you chain functions together and will launch each function as inputs
     future = f(1)
     assert future.result() == 2
 
-    future = g(f(1))
-    assert future.result() == 4
+    # Functions run concurrently, can be chained
+    f_a, f_b = f(2), f(3)
+    future = g(f_a, f_b)
+    assert future.result() == 7
 
 
 Start with the `configuration quickstart <quickstart.html#getting-started>`_ to learn how to tell Parsl how to use your computing resource,