Issue #15527: remove double parens by changing markup.

Patch by Serhiy Storchaka.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 9caebf1..5bfa351 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -329,7 +329,7 @@
 
 .. function:: setpgrp()
 
-   Call the system call :c:func:`setpgrp` or :c:func:`setpgrp(0, 0)` depending on
+   Call the system call :c:func:`setpgrp` or ``setpgrp(0, 0)`` depending on
    which version is implemented (if any).  See the Unix manual for the semantics.
 
    Availability: Unix.