Switch sphinx language to en

With sphinx release of 5.0.0, they changed default for language variable
to 'en' from None. With that current None valuable is not valid and
should not be used.

Additionally, a typo for `allowlist_externals` was fixed.

Change-Id: I25359feb4c207f174a3e3f9fa7e61076e423abbc

README.rst

@@ -19,7 +19,7 @@ The layout of this repository is::
 
   specs/<release>/
 
-You can find an example spec in `specs/template.rst`.
+You can find an example spec in `specs/templates/template.rst`.
 
 Specifications are proposed for a given release by adding them to the
 `specs/<release>` directory and posting it for review.  The implementation

doc/source/conf.py

@@ -76,7 +76,7 @@ openstackdocs_bug_tag = ''
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:

tox.ini

@@ -36,7 +36,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-allowlist_external =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf