[spec] Remove highlighting in the lists of keywords.

The highlighter does not print all keywords with the same color,
which makes sense in actual code snippets, but is confusing in the
raw lists of all keywords.

Author: sjrd

docs/_spec/01-lexical-syntax.md

@@ -83,7 +83,7 @@ For this purpose, lower case letters include not only a-z, but also all characte
 
 The following are examples of variable identifiers:
 
-> ```scala
+> ```
 >     x         maxIndex   p2p   empty_?
 >     `yield`   αρετη      _y    dot_product_*
 >     __system  _MAX_LEN_
@@ -92,7 +92,7 @@ The following are examples of variable identifiers:
 
 Some examples of constant identifiers are
 
-> ```scala
+> ```
 >     +    Object  $reserved  Džul    ǂnûm
 >     ⅰ_ⅲ  Ⅰ_Ⅲ     ↁelerious  ǃqhàà  ʹthatsaletter
 > ```
@@ -104,7 +104,7 @@ User programs should not define identifiers that contain ‘$’ characters.
 
 The following names are reserved words instead of being members of the syntactic class `id` of lexical identifiers.
 
-```scala
+```
 abstract  case      catch     class     def       do        else
 enum      export    extends   false     final     finally   for
 given     if        implicit  import    lazy      match     new
@@ -169,14 +169,14 @@ A newline in a Scala source text is treated as the special token “nl” if the
 
 The tokens that can terminate a statement are: literals, identifiers and the following delimiters and reserved words:
 
-```scala
+```
 this    null    true    false    return    type    <xml-start>
 _       )       ]       }
 ```
 
 The tokens that can begin a statement are all Scala tokens _except_ the following delimiters and reserved words:
 
-```scala
+```
 catch    else    extends    finally    forSome    match
 with    yield    ,    .    ;    :    =    =>    <-    <:    <%
 >:    #    [    )    ]    }
@@ -452,7 +452,7 @@ Characters must not necessarily be printable; newlines or other control characte
 >
 > This would produce the string:
 >
-> ```scala
+> ```
 > the present string
 >      spans three
 >      lines.
@@ -469,7 +469,7 @@ Characters must not necessarily be printable; newlines or other control characte
 >
 > evaluates to
 >
-> ```scala
+> ```
 > the present string
 > spans three
 > lines.