manual: use subheadings for primitive types

this gives us HTML anchors for each of them

doc/manual/src/expressions/language-values.md

@@ -4,7 +4,9 @@
 
 Nix has the following basic data types:
 
-  - *Strings* can be written in three ways.
+### Strings
+
+*Strings* can be written in three ways.
 
 The most common way is to enclose the string between double quotes,
 e.g., `"foo bar"`. Strings can span multiple lines. The special
@@ -112,14 +114,18 @@ Nix has the following basic data types:
 `"http://example.org/foo.tar.bz2"` can also be written as
 `http://example.org/foo.tar.bz2`.
 
-  - Numbers, which can be *integers* (like `123`) or *floating point*
+### Numbers
+
+Numbers, which can be *integers* (like `123`) or *floating point*
 (like `123.43` or `.27e13`).
 
 Numbers are type-compatible: pure integer operations will always
 return integers, whereas any operation involving at least one
 floating point number will have a floating point number as a result.
 
-  - *Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
+### Paths
+
+*Paths*, e.g., `/bin/sh` or `./builder.sh`. A path must contain at
 least one slash to be recognised as such. For instance, `builder.sh`
 is not a path: it's parsed as an expression that selects the
 attribute `sh` from the variable `builder`. If the file name is
@@ -146,9 +152,13 @@ Nix has the following basic data types:
 recognized as a path. `a.${foo}/b.${bar}` is a syntactically valid division
 operation. `./a.${foo}/b.${bar}` is a path.
 
-  - *Booleans* with values `true` and `false`.
+### Booleans
 
-  - The null value, denoted as `null`.
+*Booleans* with values `true` and `false`.
+
+### Null
+
+The null value, denoted as `null`.
 
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