fix a link to env config spec/docs (#95)

Opted for the opentelemetry.io web site which might have more stable
content locations than the github repo.

Co-authored-by: Mike Goldsmith <[email protected]>

docs/how-it-works.md

@@ -6,7 +6,7 @@ We aim to bring the automatic instrumentation experience found in languages like
 
 - No code changes required - any Go application can be instrumented without modifying the source code.
 - Support wide range of Go applications - instrumentation is supported for Go version 1.12 and above. In addition, a common practice for Go applications is to shrink the binary size by stripping debug symbols via `go build -ldflags "-s -w"`. This instrumentation works for stripped binaries as well.
-- Configuration is done via `OTEL_*` environment variables according to [OpenTelemetry Environment Variable Specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#general-sdk-configuration).
+- Configuration is done via `OTEL_*` environment variables according to [OpenTelemetry Environment Variable Specification](https://opentelemetry.io/docs/reference/specification/sdk-environment-variables/#general-sdk-configuration).
   **In order to inject instrumentation into your process, set the `OTEL_TARGET_EXE` environment variable to the path of the target executable. This is not a part of the OTEL specification mentioned above.**
 - Instrumented libraries follow the [OpenTelemetry specification](https://github.com/open-telemetry/opentelemetry-specification) and semantic conventions to produce standard OpenTelemetry data.