Rename .md sections

Author: grynspan

Documentation/ABI/TestContent.md

@@ -35,7 +35,7 @@ section in built test products:
   `MemoryLayout<UInt>.stride`. Code that walks this section must skip over this
   padding.
 
-### Record headers
+### Record layout
 
 Regardless of platform, all test content records created and discoverable by the
 testing library have the following layout:
@@ -63,7 +63,7 @@ struct SWTTestContentRecord {
 };
 ```
 
-### Record contents
+### Record content
 
 #### The kind field