Added another run time error example.

pep-0498.txt

@@ -276,8 +276,8 @@ Invalid expressions::
   SyntaxError: invalid syntax
 
 Run time errors occur when evaluating the expressions inside an
-f-string. Note that an f-string can be executed multiple times, and
-work sometimes and raise an error other times::
+f-string. Note that an f-string can be evaluated multiple times, and
+work sometimes and raise an error at other times::
 
   >>> d = {0:10, 1:20}
   >>> for i in range(3):
@@ -289,6 +289,17 @@ work sometimes and raise an error other times::
     File "<stdin>", line 2, in <module>
   KeyError: 2
 
+or::
+
+  >>> for x in (32, 100, 'fifty'):
+  ...   f'x = {x:+3}'
+  ...
+  'x = +32'
+  'x = +100'
+  Traceback (most recent call last):
+    File "<stdin>", line 2, in <module>
+  ValueError: Sign not allowed in string format specifier
+
 Leading whitespace in expressions is skipped
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