47916-parameterized-go-types.md: clean up typos and improve phrasing
47916-parameterized-go-types.md: clean up typos and improve phrasing

Change-Id: I0739388b25b22b96d508fad3b452e3e6d81aff63
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/351349
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/proposal,golang/proposal