fix getting started link in readme (#2119)

Signed-off-by: Andrew Hsu <[email protected]>

README.md

@@ -2,7 +2,7 @@
 
 <p align="center">
  <strong>
- <a href="https://opentelemetry.io/docs/collector/about/">Getting Started<a/>
+ <a href="https://opentelemetry.io/docs/collector/getting-started/">Getting Started<a/>
  &nbsp;&nbsp;&bull;&nbsp;&nbsp;
  <a href="https:/open-telemetry/community#special-interest-groups">Getting Involved<a/>
  &nbsp;&nbsp;&bull;&nbsp;&nbsp;