[FlexibleHeader] - Correct use of deprecated property in docs

[ShepJGoogle]

Update docs using old deprecated property on FlexibleHeader readme

[codecov-io]

Codecov Report

Merging #1340 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1340   +/-   ##
========================================
  Coverage    29.09%   29.09%           
========================================
  Files          137      137           
  Lines        13795    13795           
  Branches      1168     1168           
========================================
  Hits          4014     4014           
  Misses        9777     9777           
  Partials         4        4

Impacted Files	Coverage Δ
components/Slider/src/MDCSlider.m	67.75% <0%> (-0.55%)	⬇️
components/private/ThumbTrack/src/MDCThumbTrack.m	54% <0%> (+0.15%)	⬆️

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