Fix invalid format string in error message

[asottile]

Description of changes:

Found using this PR of pyflakes: PyCQA/pyflakes#443

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #4049 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #4049   +/-   ##
========================================
  Coverage    94.47%   94.47%           
========================================
  Files          187      187           
  Lines        14109    14109           
========================================
  Hits         13330    13330           
  Misses         779      779

Impacted Files	Coverage Δ
awscli/topictags.py	100% <ø> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0dab7c0...003d60c. Read the comment docs.

[asottile]

@kyleknap this is an easy 1-character change that fixes a crash:

(for example)

>>> tag = topic_name = value = 1
>>> (
... 'Tag %s for topic %s has value %. Expected a single '
... 'element in list.' % (tag, topic_name, value)
... )
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
ValueError: unsupported format character ' ' (0x20) at index 32
>>> (
... 'Tag %s for topic %s has value %s. Expected a single '
... 'element in list.' % (tag, topic_name, value)
... )
'Tag 1 for topic 1 has value 1. Expected a single element in list.'

[asottile]

@kyleknap @joguSD @stealthycoin any chance this can get merged? the programming error blocks an error message from appearing