Correct tensorflow/swift Installation readme link

[kwigginton]

This fixes the link to the Swift for TensorFlow installation instructions, as github's links are case sensitive.

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[kwigginton]

Also feel free to close and just commit this directly without CLA if it's too much of a bother, even though I've signed the CLA now

[googlebot]

CLAs look good, thanks!

[rxwei]

Thank you!