[libc][docs] Use same formatting for headers in source_layout

utils looks different from the other directory names
in the docs, see
https://libc.llvm.org/source_layout.html#the-utils-directory

Differential revision: https://reviews.llvm.org/D126211

libc/docs/source_layout.rst

@@ -84,8 +84,8 @@ toplevel ``libc`` directory itself. A test for, say the ``mmap`` function, lives
 in the directory ``test/src/sys/mman/`` as implementation of ``mmap`` lives in
 ``src/sys/mman``.
 
-The `utils` directory
----------------------
+The ``utils`` directory
+-----------------------
 
 This directory contains utilities used by other parts of the llvm-libc system.
 See the `README` files, in the sub-directories within this directory, to learn