Fix broken headings in Markdown files

[bryant1410]

GitHub changed the way Markdown headings are parsed, so this change fixes it.

See bryant1410/readmesfix for more information.

Tackles bryant1410/readmesfix#1

[codecov-io]

Codecov Report

Merging #23 into master will decrease coverage by 0.08%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #23      +/-   ##
============================================
- Coverage     69.79%   69.71%   -0.09%     
  Complexity      403      403              
============================================
  Files            68       68              
  Lines          3447     3447              
  Branches        374      374              
============================================
- Hits           2406     2403       -3     
- Misses          933      936       +3     
  Partials        108      108

Impacted Files	Coverage Δ	Complexity Δ
...en/rx/internal/operators/OperatorBufferToFile.java	82.17% <0%> (-2.33%)	4% <0%> (ø)

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[davidmoten]

Thanks!