Use path, not external URL, for link into glossary (#3375)

- Contributes to
open-telemetry/opentelemetry.io#2429
- This is part of an effort to normalize links, for improved link
checking on the OTel website

specification/common/attribute-requirement-level.md

@@ -19,7 +19,7 @@
 
 _This section applies to Log, Metric, Resource, and Span, and describes requirement levels for attributes defined in semantic conventions._
 
-Attribute requirement levels apply to the [instrumentation library](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/glossary.md#instrumentation-library).
+Attribute requirement levels apply to the [instrumentation library](../glossary.md#instrumentation-library).
 
 The following attribute requirement levels are specified: