doc: services: posix: _POSIX_THREAD_PRIO_INHERIT unsupported

_POSIX_THREAD_PRIO_INHERIT was mistakenly marked supported in
commit a07774eada presumably because we have prototypes for the
functions below.

pthread_mutexattr_getprotocol()
pthread_mutexattr_setprotocol()

However, the functions are not actually implemented in Zephyr.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>
diff --git a/doc/services/portability/posix.rst b/doc/services/portability/posix.rst
index 84e0ea2..edc384f 100644
--- a/doc/services/portability/posix.rst
+++ b/doc/services/portability/posix.rst
@@ -97,7 +97,7 @@
     _POSIX_THREAD_ATTR_STACKADDR,yes
     _POSIX_THREAD_ATTR_STACKSIZE,yes
     _POSIX_THREAD_CPUTIME,
-    _POSIX_THREAD_PRIO_INHERIT,yes
+    _POSIX_THREAD_PRIO_INHERIT,
     _POSIX_THREAD_PRIO_PROTECT,
     _POSIX_THREAD_PRIORITY_SCHEDULING,yes
     _POSIX_THREAD_SPORADIC_SERVER,