[libc][docs] Fix libc docs build post llvm#129138

The docs build action was failing with libc due to checks.rst not
existing in the expected path. This patch adjusts the path to the actual
path which seems to make everything happy. It seems like this did not
show up before as stdfix.rst was not included in a place that actually
caused it to get picked up by sphinx.

Author: boomanaiden154

libc/docs/headers/stdfix.rst

@@ -2,7 +2,7 @@
 StdFix Functions
 ================
 
-.. include:: ../../check.rst
+.. include:: ../check.rst
 
 Standards and Goals
 -------------------