PEP 525: Fix comment in code example (#4110)

peps/pep-0525.rst

@@ -339,7 +339,7 @@ The following methods and properties are defined:
 
        v = await g.athrow(ZeroDivisionError)
        print(v)                # Will print 'world' after
-                               $ sleeping 0.2 seconds.
+                               # sleeping 0.2 seconds.
 
 5. ``agen.aclose()``: Returns an *awaitable*, that throws a
    ``GeneratorExit`` exception into the generator.  The *awaitable* can