add ML.NET to CODEOWNERS (#5478)

Author: mairaw

.github/CODEOWNERS

@@ -33,6 +33,9 @@
 # The Visual Basic Guide:
 /docs/visual-basic/** @rpetrusha
 
+# The ML.NET Guide:
+/docs/machine-learning/** @JRAlexander
+
 ############### .NET Core ########################
 # Docker
 /docs/core/docker/** @JRAlexander
@@ -62,4 +65,3 @@
 /docs/standard/io/** @mairaw
 # .NET base types - Formatting
 /docs/standard/base-types/formatting-types/** @rpetrusha
-