Update BUILD.gn (#23770)

Fixed typo in assert message.
diff --git a/src/pybindings/pycontroller/BUILD.gn b/src/pybindings/pycontroller/BUILD.gn
index c4dce51..b15e424 100644
--- a/src/pybindings/pycontroller/BUILD.gn
+++ b/src/pybindings/pycontroller/BUILD.gn
@@ -50,7 +50,7 @@
   } else if (current_os == "linux") {
     include_dirs += [ "/usr/include/python3.9" ]
   } else {
-    assert(false, "OS not supprted.")
+    assert(false, "OS not supported.")
   }
 
   sources = [