[grpc-js] Fix error messages in serviceConfig validation

[matthewbinshtok]

This PR corrects five error messages in grpc-js's serviceConfig validation:

• 3 typos corrected from Invld to Invalid
• 2 corrected to say that strings containing decimals are valid in initialBackoff and maxBackoff values of retryConfig.

Validation for initialBackoff and maxBackoff in retryConfig use the following regex which allows decimals:

/**
 * Recognizes a number with up to 9 digits after the decimal point, followed by
 * an "s", representing a number of seconds.
 */
const DURATION_REGEX = /^\d+(\.\d{1,9})?s$/;

However, the error messages for initialBackoff and maxBackoff in validateRetryPolicy state that the values "must be a string consisting of a positive integer followed by s".

I discovered this when correcting a copied Go serviceConfig containing decimals that did not start with a 0:

    ...
    "InitialBackoff": ".002s",
    "MaxBackoff": ".01s",

becomes

    ...
    "initialBackoff": "0.002s",
    "maxBackoff": "0.01s",

[linux-foundation-easycla]

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• ✅ login: matthewbinshtok / name: Matthew Binshtok (d6925d9, fc5bee7)

[matthewbinshtok]

Test suites failed due to error message comparison, I've changed the tests to expect the new error message.

[matthewbinshtok]

Latest test failures seem unrelated.