Fix sed command in RTD config

The build process on readthedocs seems to use some strange parsing logic
because the command `sed -i '/gpg/d;/notmuch2/d' pyproject.toml`
resulted in an error:
~~~
sed: no input files
/bin/sh: 1: /notmuch2/d: not found
~~~

readthedocs.yaml

@@ -12,7 +12,7 @@ build:
   jobs:
     # remove and mock problematic dependencies
     pre_install:
-      - sed -i '/gpg/d;/notmuch2/d' pyproject.toml
+      - sed -i -e /gpg/d -e /notmuch2/d pyproject.toml
       - touch gpg.py
       - echo NullPointerError = NotmuchError = None > notmuch2.py
     # make the git state clean again for setuptools_scm