git.txt: fix heading line of tildes

The two-line heading added in 8525e92 (Document HOME environment
variable, 2024-12-09) uses too many tilde characters, so the heading
isn't detected as such. Both AsciiDoc and Asciidoctor end up
misrendering this in different ways.

Use the correct number of tilde characters to fix this.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -478,7 +478,7 @@ their values the same way as Boolean valued configuration variables, e.g.
 Here are the variables:
 
 System
-~~~~~~~~~~~~~~~~~~
+~~~~~~
 `HOME`::
 	Specifies the path to the user's home directory. On Windows, if
 	unset, Git will set a process environment variable equal to: