Fixes in README

[sreeram-venkitesh]

Summary of change in 50 characters or less

Fixes in the README.md file. Updated `cel.Declarations` to `cel.Variable` since the code snippet has been updated and the accompanying doc doesn't seem to have been updated. Minor grammatical fixes. 

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[sreeram-venkitesh]

@TristonianJones Can you please take a look? Thanks!

[TristonianJones]

/gcbrun

[TristonianJones]

Thanks @sreeram-venkitesh!