Fixed some tiny grammars in docs

[ksohan]

Fixed some tiny grammar mistakes in documents.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.

[codecov]

Codecov Report

Merging #1706 (c09ee8a) into master (42dc88e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1706   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           51        51           
  Lines         3417      3417           
=========================================
  Hits          3417      3417           

Impacted Files	Coverage Δ
autokeras/engine/tuner.py	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 42dc88e...c09ee8a. Read the comment docs.