Fixed broken link to BasePlugin (#1270)

* Fixed broken link to BasePlugin

Updated the link to the BasePlugin.

* Update base plugin link

I should probably include the correct link if I'm fixing the link...

doc/plugin-guide.md

@@ -46,7 +46,7 @@ Please refer to the [HTTP instrumentation][http-plugin] or [gRPC instrumentation
 
 [shimmer]: https://github.com/othiym23/shimmer
 [builtin-plugins]: https://github.com/open-telemetry/opentelemetry-js#plugins
-[base-plugin]: https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-core/src/trace/instrumentation/BasePlugin.ts#L29
+[base-plugin]: https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-core/src/platform/node/BasePlugin.ts
 [http-plugin]: https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-plugin-http/src/http.ts#L44
 [grpc-plugin]: https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-plugin-grpc/src/grpc.ts#L52
 [DEFAULT_INSTRUMENTATION_PLUGINS]: https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-node/src/config.ts#L29