Update src/membership.md

Co-authored-by: Jane Lusby <jlusby@yaah.dev>

Author: m-ou-se

src/membership.md

@@ -17,7 +17,7 @@ and have already been involved in relevant library work.
 In all cases, the process of adding a new members goes as follows:
 
 1. A member of the Library (API) Team proposes the addition of a contributor on our private mailing list.
-   This includes proposal includes:
+   This proposal includes:
    - A short description of what this person has been working on; how they have been contributing.
    - A few specific examples of cases where this person clearly communicated their ideas.
    - A few specific examples that show this person understands what are and what aren't acceptable changes.\