doc: Clarify os.urandom return type (GH-30282)

Other descriptions in the same file also use 'bytestring' to refer to bytes objects
(cherry picked from commit 4b99803)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

Doc/library/os.rst

@@ -4869,7 +4869,7 @@ Random numbers
 
 .. function:: urandom(size)
 
-   Return a string of *size* random bytes suitable for cryptographic use.
+   Return a bytestring of *size* random bytes suitable for cryptographic use.
 
    This function returns random bytes from an OS-specific randomness source.  The
    returned data should be unpredictable enough for cryptographic applications,