Fix string formatting in pulse drawer

[Randl]

Summary

This a small fix to the formatting, which used an expression of form "%s" % time * dt. Since the % has higher priority it end up multiplying string by floating point number dt. I think the intent was to use time * dt for formatting

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[coveralls]

Pull Request Test Coverage Report for Build 4111690264

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.277%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/visualization/pulse/matplotlib.py	0	1	0.0%

Totals
Change from base Build 4107383485:	0.0%
Covered Lines:	67250
Relevant Lines:	78861

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💛 - Coveralls

[HuangJunye]

@Randl Thanks for your contribution. Can you please provide some context for this fix. What is the current behavior and what it is after the fix?

[Randl]

The following is minimal example:

from qiskit.pulse.channels import DriveChannel
from qiskit.pulse.instructions import ShiftPhase
phase = 10
channel = DriveChannel(0)
ShiftPhase(phase, channel).draw(table=True, channels=[channel], dt=2)

It will crush if dt is non-integer, and otherwise will print repeated t=0 string in the table dt times

[woodsp-ibm]

>>> "t = %s" % 2 * 5
't = 2t = 2t = 2t = 2t = 2'
>>> "t = %s" % 2 * 5.5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't multiply sequence by non-int of type 'float'


>>> "t = %s" % (2 * 5)
't = 10'
>>> "t = %s" % (2 * 5.5)
't = 11.0'

[1ucian0]

We need to extend our visual tests for capturing this. In the meanwhile, let's merge it. Thanks!