[test] Fix documentation of %{fs-sep} et al (#95088)

Author: jayfoad

llvm/docs/TestingGuide.rst

@@ -741,16 +741,16 @@ RUN lines:
 
    Expands to the path separator, i.e. ``:`` (or ``;`` on Windows).
 
-``${fs-src-root}``
+``%{fs-src-root}``
    Expands to the root component of file system paths for the source directory,
    i.e. ``/`` on Unix systems or ``C:\`` (or another drive) on Windows.
 
-``${fs-tmp-root}``
+``%{fs-tmp-root}``
    Expands to the root component of file system paths for the test's temporary
    directory, i.e. ``/`` on Unix systems or ``C:\`` (or another drive) on
    Windows.
 
-``${fs-sep}``
+``%{fs-sep}``
    Expands to the file system separator, i.e. ``/`` or ``\`` on Windows.
 
 ``%/s, %/S, %/t, %/T``