BTS_Tests: rewrite outdated comment

We are no longer "appending" imsi digits. This is now done properly
using a struct but let's still mention what we use the IMSI for.

Related: OS#5927
Change-Id: I3d943cd96e1d9627ad68e3439b2a649baa5785f1
---
M bts/BTS_Tests.ttcn
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 63a9a05..d39bb77 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -5847,7 +5847,7 @@
 	f_init_l1ctl();
 	f_l1_tune(L1CTL);
 
-	/* append 3 last imsi digits so BTS can compute pagng group */
+	/* The BTS will use the last three IMSI digits to compute paging group. */
 	f_PCUIF_tx_imm_ass_pch(PCU, g_pcu_conn_id, imm_ass, '123459987'H);
 
 	timer T := 0.5;

