docs (en.md): move the split vowels section into the short vowels section.

docs/languages/gmw/en.md

   - [Rhotic Vowels](#rhotic-vowels)
   - [Reduced Vowels](#reduced-vowels)
   - [Diphthongs](#diphthongs)
-  - [Split Vowels](#split-vowels)
 - [References](#references)
 
 ----------
 | STRUT       | `V`   | ʌ     | ʌ     |
 | FOOT        | `U`   | ʊ     | ʊ     |
 
+Additionally, Wells defines the following lexical sets to describe vowels that
+are different in both RP and GenAm:
+
+| Lexical Set | en    | RP    | GenAm |
+|-------------|-------|-------|-------|
+| BATH        | `aa`  | ɑː    | æ     |
+| CLOTH       | `O2`  | ɒ     | ɔ     |
+
 ### Long Vowels
 
 | Lexical Set | en    | RP    | GenAm |
 | GOAT        | `oU`  | əʊ̯    | oʊ̯    |
 | MOUTH       | `aU`  | aʊ̯    | aʊ̯    |
 
-### Split Vowels
-
-These are lexical sets defined by John Wells that are merged with other lexical
-sets in both RP and GenAm, so have split from one of those lexical sets and
-merged with the other.
-
-| Lexical Set | en    | RP    | GenAm |
-|-------------|-------|-------|-------|
-| BATH        | `aa`  | ɑː    | æ     |
-| CLOTH       | `O2`  | ɒ     | ɔ     |
-
 ## References
 
 1. <a name="ref1"></a> Wikipedia.