doc: replace 3 dash with correct 2 dash in git-config(1)

Commit 4e51389 (builtin/config: introduce "get" subcommand, 2024-05-06)
introduced this typo.  It uses 3 dashes for regexp argument instead of
correct 2 dashes.

Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: CoelacanthusHex

Documentation/git-config.txt

@@ -130,7 +130,7 @@ OPTIONS
 --all::
 	With `get`, return all values for a multi-valued key.
 
----regexp::
+--regexp::
 	With `get`, interpret the name as a regular expression. Regular
 	expression matching is currently case-sensitive and done against a
 	canonicalized version of the key in which section and variable names