Added 'w3c' to valid JAEGER_PROPAGATION options

[chgl]

Which problem is this PR solving?

Resolves #733

Short description of the changes

• added w3c option to docs

[codecov]

Codecov Report

Merging #736 into master will decrease coverage by 0.31%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #736      +/-   ##
============================================
- Coverage     89.08%   88.77%   -0.32%     
+ Complexity      601      598       -3     
============================================
  Files            73       73              
  Lines          2253     2253              
  Branches        292      292              
============================================
- Hits           2007     2000       -7     
- Misses          155      161       +6     
- Partials         91       92       +1     

Impacted Files	Coverage Δ	Complexity Δ
...java/io/jaegertracing/zipkin/ZipkinV2Reporter.java	71.42% <0.00%> (-28.58%)	2.00% <0.00%> (-1.00%)
...rtracing/internal/reporters/CompositeReporter.java	71.42% <0.00%> (-28.58%)	6.00% <0.00%> (-1.00%)
...gertracing/internal/reporters/LoggingReporter.java	81.81% <0.00%> (-9.10%)	4.00% <0.00%> (-1.00%)

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[yurishkuro]

Thanks