doc: update LTS2 version in docs

LTS is now 2.7.5.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/conf.py b/doc/conf.py
index 3fc1705..72263cf 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -164,7 +164,7 @@
         ("3.2.0", "/3.2.0/"),
         ("3.1.0", "/3.1.0/"),
         ("3.0.0", "/3.0.0/"),
-        ("2.7.4 (LTS)", "/2.7.4/"),
+        ("2.7.5 (LTS)", "/2.7.5/"),
     ),
     "display_vcs_link": True,
     "reference_links": {