Docs: Fix Sphinx version in getting started

Update Sphinx version to 2.0.1 to align with that in requirements.txt.

Change-Id: I3420e823426d4c7cbb83271ddf0a1a974f7ae9f1
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/getting_started/tfm_getting_started.rst b/docs/getting_started/tfm_getting_started.rst
index b6c52a4..1fddbb0 100644
--- a/docs/getting_started/tfm_getting_started.rst
+++ b/docs/getting_started/tfm_getting_started.rst
@@ -356,7 +356,7 @@
    "click",,"Firmware"
    "imgtool",,"Firmware"
    "Doxygen",">1.8","Reference manual"
-   "Sphinx","=1.7.9","User Guide"
+   "Sphinx","=2.0.1","User Guide"
    "sphinxcontrib-plantuml",,"User Guide"
    "sphinxcontrib-svg2pdfconverter",,"User Guide"
    "sphinx-rtd-theme",,"User Guide"