Use correct IM_MODULE variables for setting up scim

At the moment, the readme file for scim asks users to set QT_IM_MODULE
and GTK_IM_MODULE to "scim", which is incorrect and prevent scim from
being activated. To make scim usable, they should be set to "xim".

Tested on OpenBSD 7.0 GENERIC.MP#223 @amd64

This problem is reported by f5b from gobsd.org

Index: README
===================================================================
RCS file: /cvs/ports/inputmethods/scim/pkg/README,v
retrieving revision 1.3
diff -u -r1.3 README
--- README 4 Sep 2018 12:46:14 -0000 1.3
+++ README 22 Sep 2021 22:38:20 -0000
@@ -7,8 +7,8 @@
To make SCIM work when X is started add the following lines to
~/.xinitrc or ~/.xsession:

- export GTK_IM_MODULE="scim"
- export QT_IM_MODULE="scim"
+ export GTK_IM_MODULE="xim"
+ export QT_IM_MODULE="xim"
export XMODIFIERS="@im=SCIM"
${TRUEPREFIX}/bin/scim -d