OsmoGSMTester: Update suite.conf example

This is the one we are usig nowadays in suite.conf, which explicitly
asks to be run with modems supporting sms features.

Change-Id: If5dbc3b4689b282db6fae4668e2838be59e88da8
diff --git a/doc/manuals/chapters/intro.adoc b/doc/manuals/chapters/intro.adoc
index 8c106b7..6e377da 100644
--- a/doc/manuals/chapters/intro.adoc
+++ b/doc/manuals/chapters/intro.adoc
@@ -316,6 +316,8 @@
   - times: 1
   modem:
   - times: 2
+    features:
+    - sms
 ----
 
 It may also request e.g. specific BTS models, but this is typically left to