Correct typos in model_reference comments

Signed-off-by: Pierre R. Mai <pmai@pmsf.de>

Author: hschoen

osi_groundtruth.proto

@@ -167,8 +167,8 @@ message GroundTruth
     //
     // \note It is implementation-specific how model_references are resolved to
     // 3d models. The parts the world model contains are also implementation-specific.
-    // For example, the world model can wether contain street geometries or
-    // derives street geometies automatically from a map reference.
+    // For example, the world model can either contain street geometries or
+    // derives street geometries automatically from a map reference.
     //
     optional string model_reference = 16;
 }