Update sample decision name based on spec update

[ThomsonTan]

Inconsistent naming of sampling decision was updated in OTel trace SDK spec PR, this PR does the renaming to keep conformance with spec.

[codecov]

Codecov Report

Merging #327 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files         148      148           
  Lines        6651     6651           
=======================================
  Hits         6292     6292           
  Misses        359      359           

Impacted Files	Coverage Δ
sdk/include/opentelemetry/sdk/trace/sampler.h	100.00% <ø> (ø)
.../opentelemetry/sdk/trace/samplers/parent_or_else.h	100.00% <ø> (ø)
...ude/opentelemetry/sdk/trace/samplers/probability.h	100.00% <ø> (ø)
sdk/test/trace/tracer_test.cc	99.09% <ø> (ø)
...lude/opentelemetry/sdk/trace/samplers/always_off.h	100.00% <100.00%> (ø)
sdk/src/trace/samplers/parent_or_else.cc	100.00% <100.00%> (ø)
sdk/src/trace/samplers/probability.cc	100.00% <100.00%> (ø)
sdk/src/trace/tracer.cc	70.96% <100.00%> (ø)
sdk/test/trace/always_off_sampler_test.cc	100.00% <100.00%> (ø)
sdk/test/trace/parent_or_else_sampler_test.cc	100.00% <100.00%> (ø)
... and 1 more