Give instances of `MasterTask` in doc examples less sensitive names

[meliache]

I already mentioned the rationale in #117 . I think it would be more appropriate to use other task names than MasterTask, which doesn't convey much meaning anyway. In one instance I named it MainTask, similar to how we renamed the main branch name in #39. But since we are implementing luigi.WrapperTask, it might be more meaningful to encode what is being wrapped in the name, e.g. AnalysisWrapperTask, which I used in the other two occurances.

This is just a documentation change, but I think it is important because the examples are how we present b2luigi, it's the first of b2luigi that new users see and often they just adopt the naming from the examples.

It's not urgent, so can wait for a comment from @nils-braun who did the branch renaming in #39, maybe you have some ideas with regards to proper naming.

[codecov-commenter]

Codecov Report

Merging #118 (752920b) into main (31fe325) will increase coverage by 0.13%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   56.58%   56.72%   +0.13%     
==========================================
  Files          23       23              
  Lines        1495     1495              
==========================================
+ Hits          846      848       +2     
+ Misses        649      647       -2     

Impacted Files	Coverage Δ
b2luigi/core/utils.py	73.15% <0.00%> (+1.05%)	:arrow_up:

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