Fix wording in Mono.expand documentation

[martin-tarjanyi]

Noticed after reading blog post by @bsideup :)

[pivotal-issuemaster]

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[simonbasle]

thanks for the patch 👍

[simonbasle]

@pivotal-issuemaster this is an obvious fix

[pivotal-issuemaster]

@martin-tarjanyi This Pull Request contains an obvious fix. Signing the Contributor License Agreement is not necessary.

[codecov-io]

Codecov Report

Merging #1999 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #1999      +/-   ##
===========================================
- Coverage     81.73%   81.7%   -0.03%     
- Complexity     4014    4015       +1     
===========================================
  Files           375     375              
  Lines         30926   30926              
  Branches       5808    5808              
===========================================
- Hits          25277   25269       -8     
- Misses         4052    4056       +4     
- Partials       1597    1601       +4

Impacted Files	Coverage Δ	Complexity Δ
...ore/src/main/java/reactor/core/publisher/Mono.java	91.03% <ø> (ø)	273 <0> (ø)	⬇️
...in/java/reactor/core/publisher/FluxWindowWhen.java	84.13% <0%> (-1.93%)	2% <0%> (ø)
...main/java/reactor/core/publisher/FluxIterable.java	77.14% <0%> (-0.72%)	12% <0%> (ø)
...rc/main/java/reactor/core/publisher/Operators.java	79.27% <0%> (-0.7%)	127% <0%> (ø)
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.57% <0%> (-0.38%)	19% <0%> (ø)
...ain/java/reactor/core/publisher/FluxPublishOn.java	86.57% <0%> (+0.2%)	6% <0%> (ø)	⬇️
...ain/java/reactor/core/publisher/FluxConcatMap.java	89.72% <0%> (+0.27%)	7% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxReplay.java	84.29% <0%> (+0.45%)	28% <0%> (+1%)	⬆️

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