cosmetic: OsmoGSMTester: fix dir name

All other subdirs are without dashes, but I added Osmo-GSM-Tester with dashes.
Comply with the naming scheme and remove dashes.

Change-Id: I36d0c94dde5deffba04b27436ae499a42c519bec
diff --git a/doc/manuals/chapters/test_api.adoc b/doc/manuals/chapters/test_api.adoc
new file mode 100644
index 0000000..cabde4c
--- /dev/null
+++ b/doc/manuals/chapters/test_api.adoc
@@ -0,0 +1,3 @@
+== Test API
+
+*TODO*