Fix note markup (#16805).

Patch by Tshepang Lekhonkhobe, reviewed by Georg Brandl.
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 705509f..4dad2c8 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -548,6 +548,7 @@
    .. index:: module: _thread
 
    .. note::
+
       When only the main thread exists, no GIL operations are needed. This is a
       common situation (most Python programs do not use threads), and the lock
       operations slow the interpreter down a bit. Therefore, the lock is not