go2draft-contracts.md: add missing period

Change-Id: Ibde4ea7ab98d795c1504961d02d25ef4b4b1c12e
GitHub-Last-Rev: 020bcd096aa288cc2314f4e14d87b5fe0cef523d
GitHub-Pull-Request: golang/proposal#20
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/187961
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/design/go2draft-contracts.md b/design/go2draft-contracts.md
index 10fd074..69838c7 100644
--- a/design/go2draft-contracts.md
+++ b/design/go2draft-contracts.md
@@ -1658,7 +1658,7 @@
   to support type arguments that do not already satisfy the contract,
   such as, for example, defining an `==` operator in terms of an
   `Equal` method, or vice-versa.
-* No parameterization on non-type values such as constants
+* No parameterization on non-type values such as constants.
   This arises most obviously for arrays, where it might sometimes be
   convenient to write `type Matrix(type n int) [n][n]float64`.
   It might also sometimes be useful to specify significant values for