Documentation/kernel-parameters: Fix a reference to vga-softcursor.rst

A very minor oversight that dates all the way back to rst migration in
commit 9d85025 ("docs-rst: create an user's manual book").

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241231190240.417446-1-lkundrak@v3.sk

Documentation/admin-guide/kernel-parameters.txt

@@ -7469,7 +7469,7 @@
 	vt.cur_default=	[VT] Default cursor shape.
 			Format: 0xCCBBAA, where AA, BB, and CC are the same as
 			the parameters of the <Esc>[?A;B;Cc escape sequence;
-			see VGA-softcursor.txt. Default: 2 = underline.
+			see vga-softcursor.rst. Default: 2 = underline.
 
 	vt.default_blu=	[VT]
 			Format: <blue0>,<blue1>,<blue2>,...,<blue15>