docs: Fix footnote after r260042.

r260042 removed a footnote referring to autoconf, but it left
around one item still referring to that footnote (libtool), and
it didn't renumber the later footnote reference.

llvm-svn: 264663
diff --git a/llvm/docs/GettingStarted.rst b/llvm/docs/GettingStarted.rst
index 5638144..6833317 100644
--- a/llvm/docs/GettingStarted.rst
+++ b/llvm/docs/GettingStarted.rst
@@ -202,8 +202,7 @@
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor
 `GCC <http://gcc.gnu.org/>`_                                >=4.7.0      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=2.7        Automated test suite\ :sup:`2`
-`libtool <http://savannah.gnu.org/projects/libtool>`_       1.5.22       Shared library manager\ :sup:`3`
-`zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`4`
+`zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 =========================================================== ============ ==========================================
 
 .. note::