fix wrong method

[buzzerrookie]

Description

Fix wrong method in doc jmeter_tutorial.html.

Motivation and Context

The one who wants to extend Jmeter UI will not be misled.

How Has This Been Tested?

No need to test.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[codecov-io]

Codecov Report

Merging #471 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #471      +/-   ##
============================================
- Coverage     53.63%   53.62%   -0.01%     
+ Complexity     9377     9376       -1     
============================================
  Files          1000     1000              
  Lines         61467    61467              
  Branches       6921     6921              
============================================
- Hits          32966    32964       -2     
- Misses        26093    26094       +1     
- Partials       2408     2409       +1

Impacted Files	Coverage Δ	Complexity Δ
...s/org/apache/jmeter/timers/PoissonRandomTimer.java	72.97% <0%> (-5.41%)	9% <0%> (-1%)

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