fixed wrong example for loop controller iteration variable

[ptrd]

Description

Fixed small (but confusing) error in documentation.

[codecov-io]

Codecov Report

Merging #377 into trunk will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##              trunk     #377      +/-   ##
============================================
+ Coverage     58.48%   58.49%   +<.01%     
  Complexity    10375    10375              
============================================
  Files          1174     1174              
  Lines         74600    74600              
  Branches       7255     7255              
============================================
+ Hits          43632    43635       +3     
+ Misses        28471    28467       -4     
- Partials       2497     2498       +1

Impacted Files	Coverage Δ	Complexity Δ
...s/org/apache/jmeter/timers/PoissonRandomTimer.java	72.97% <0%> (-5.41%)	9% <0%> (-1%)
...ocol/jms/org/apache/jmeter/protocol/jms/Utils.java	56.98% <0%> (+2.15%)	15% <0%> (ø)	⬇️
...mpler/hc/JMeterPoolingClientConnectionManager.java	29.83% <0%> (+2.41%)	11% <0%> (+1%)	⬆️

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[FSchumacher]

Thanks for the contribution.