fix link to opentelemetry exporters (#1211)

Author: paulroche

docs/index.rst

@@ -30,7 +30,7 @@ In addition, there are several extension packages which can be installed separat
     pip install opentelemetry-instrumentation-{instrumentation}
 
 These are for exporter and instrumentation packages respectively.
-The packages can be found in :scm_web:`instrumentation/ directory of the repository <ext/>`.
+The packages can be found in :scm_web:`instrumentation/ directory of the repository <exporter/>`.
 
 Extensions
 ----------