[Doc] rename handler to adapter

[bojiang]

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Documentation
• Test, CI, or build
• None

Components (if applicable)

• BentoService (model packaging, dependency management, handler definition)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, logging, metrics, tracing, benchmark, OpenAPI)
• YataiService (model management, deployment automation)
• Documentation

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change in bentoml/gallery example notebooks
• I have sent a pull request to bentoml/gallery to make that change

[codecov]

Codecov Report

Merging #789 into master will decrease coverage by 0.45%.
The diff coverage is 65.21%.

@@            Coverage Diff             @@
##           master     #789      +/-   ##
==========================================
- Coverage   55.53%   55.07%   -0.46%     
==========================================
  Files         108      114       +6     
  Lines        8144     8241      +97     
==========================================
+ Hits         4523     4539      +16     
- Misses       3621     3702      +81     

Impacted Files	Coverage Δ
bentoml/__init__.py	100.00% <ø> (ø)
bentoml/adapters/pytorch_tensor_input.py	0.00% <0.00%> (ø)
bentoml/artifact/fastai_model_artifact.py	43.58% <ø> (-1.42%)	⬇️
bentoml/artifact/fasttext_model_artifact.py	41.37% <ø> (-1.96%)	⬇️
bentoml/artifact/h2o_model_artifact.py	39.39% <ø> (-1.79%)	⬇️
bentoml/artifact/keras_model_artifact.py	20.47% <ø> (-0.63%)	⬇️
bentoml/artifact/lightgbm_model_artifact.py	38.23% <ø> (-1.77%)	⬇️
bentoml/artifact/onnx_model_artifact.py	84.93% <ø> (-0.21%)	⬇️
bentoml/artifact/pytorch_model_artifact.py	85.00% <ø> (-0.37%)	⬇️
bentoml/artifact/sklearn_model_artifact.py	70.00% <ø> (-0.74%)	⬇️
... and 93 more

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