Update calculated expression example to calculate age from date of birth

[jingtang10]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Description
Update calculated expression example to calculate age from date of birth.

See discussion https://chat.fhir.org/#narrow/stream/179266-fhirpath/topic/Calculating.20the.20period.20between.20two.20dates

The FHIRPath expression uses string manipulation to get the years of today's date and the date of birth, and calculate the age by substraction. It also compares the month and date parts of today's date and the date of birth to figure out if an additional year needs to be added to the result.

Alternative(s) considered
NA

Type
Bug fix

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[jingtang10]

Video:

age4.mp4

[aditya-07]

LGTM 👍