Fix minor typo in 3681-default-field-values.md

Fixing a one-letter typo in RFC 3681.

text/3681-default-field-values.md

@@ -1063,7 +1063,7 @@ However, it has some notable problems:
 + It provides zero improvements to the ergonomics of *specifying* defaults,
   only for using them. Arguably, the most important aspect of this RFC is
   not the syntax `Foo { .. }` but rather the ability to provide default values
-  for field.
+  for fields.
 
 + By extension, the improvement to documentation clarity is lost.