Remove support to allow_different_nesting, from markdownlint (#1248)

* Remove support to allow_different_nesting, from markdownlint

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

* Update specification/metrics/semantic_conventions/README.md

Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>

* Run markdown-toc

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

Co-authored-by: Armin Ruech <armin.ruech@dynatrace.com>

.markdownlint.yaml

@@ -1,6 +1,5 @@
 {
   "default": true,
-  "MD024": { "allow_different_nesting": true },
   "MD029": { "style": "ordered" },
   "ul-style": false,  # MD004
   "line-length": false,  # MD013

specification/metrics/semantic_conventions/README.md

@@ -9,7 +9,7 @@
   * [Pluralization](#pluralization)
 - [General Metric Semantic Conventions](#general-metric-semantic-conventions)
   * [Instrument Naming](#instrument-naming)
-  * [Units](#units-1)
+  * [Instrument Units](#instrument-units)
 
 <!-- tocstop -->
 
@@ -137,7 +137,7 @@ freely. For example, `system.paging.faults` and `system.network.packets`.
 Units do not need to be specified in the names since they are included during
 instrument creation, but can be added if there is ambiguity.
 
-### Units
+### Instrument Units
 
 Units should follow the [UCUM](http://unitsofmeasure.org/ucum.html) (need
 more clarification in