add comma.

Author: gottesmm

proposals/0000-compiler-settings.md

@@ -85,7 +85,7 @@ feature should be accomplished by:
 3. Updating the language version of the module so from that point on warnings
    will become errors to prevent backsliding.
 
-This creates developer friction when applied to large modules since the size of
+This creates developer friction when applied to large modules, since the size of
 the module makes it difficult to eliminate all of the warnings at once. This
 results in either the module being updated over time on main during which main
 is left in an intermediate, partially updated state or the module being updated