docs: update "requests>=2.20.0"

To resolve the security alert https://nvd.nist.gov/vuln/detail/CVE-2018-18074.

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

docs/requirements.txt

@@ -12,7 +12,7 @@ MarkupSafe==1.0
 Pygments==2.2.0
 pytz==2017.2
 PyYAML==3.12
-requests==2.18.3
+requests>=2.20.0
 six==1.10.0
 snowballstemmer==1.2.1
 Sphinx==1.6.3