Fixes PEP 505 grammar (#699)

pep-0505.rst

@@ -62,10 +62,9 @@ The following rules of the Python grammar are updated to read::
 
     augassign: ('+=' | '-=' | '*=' | '@=' | '/=' | '%=' | '&=' | '|=' | '^=' |
                 '<<=' | '>>=' | '**=' | '//=' | '??=')
-    term: coalesce (('*'|'@'|'/'|'%'|'//') coalesce)*
-    coalesce: factor ('??' factor)*
 
-    # factor eventually resolved to atom_expr
+    power: coalesce ['**' factor]
+    coalesce: atom_expr ['??' factor]
     atom_expr: ['await'] atom trailer*
     trailer: ('(' [arglist] ')' |
               '[' subscriptlist ']' |
@@ -83,8 +82,8 @@ determined to be ``None``. For example::
     def c(): return None
     def ex(): raise Exception()
 
-    (a ?? 2 ** b ?? 3) == a ?? (2 ** b) ?? 3
-    (a * b ?? c // d) == (a * b) ?? (c // d)
+    (a ?? 2 ** b ?? 3) == a ?? (2 ** (b ?? 3))
+    (a * b ?? c // d) == a * (b ?? c) // d
     (a ?? True and b ?? False) == (a ?? True) and (b ?? False)
     (c() ?? c() ?? True) == True
     (True ?? ex()) == True