[Doc] Fixed grammatical typos in README

[alxwen711]

Description

There were a few grammatical typos in the README; exact edits are listed in the changes section.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• To the best of my knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change

Changes

A GPU-ready graph library

It bundles structural data as well as features for a better control.

Easy to learn and use

DGL provides a plenty of learning materials for all kinds of users from ML researcher researchers to domain experts.

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 1847184

Build ID: 1

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[dgl-bot]

Commit ID: 2c1f124

Build ID: 2

Status: ✅ CI test succeeded

Report path: link

Full logs path: link

[jermainewang]

Thanks for the fix!