br,docs: fix some typos

[cuishuang]

What problem does this PR solve?

Fix some typos in br/* and docs/design/2019-11-05-index-advisor.md

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Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/519106134f347edec5c2bbb69c32e800eb4fca0b

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.7607%. Comparing base (2ae8529) to head (5191061).
Report is 11185 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                @@
##             master     #32710         +/-   ##
=================================================
- Coverage   77.1798%   63.7607%   -13.4192%     
=================================================
  Files           588        773        +185     
  Lines        148465     258457     +109992     
=================================================
+ Hits         114585     164794      +50209     
- Misses        23589      80523      +56934     
- Partials      10291      13140       +2849