GHC-39999: add missing quotes

message-index/messages/GHC-39999/adding-number-to-string/index.md

@@ -20,6 +20,6 @@ Main.hs:1:16: error: [GHC-39999]
 
 Given that Haskell is a strong statically typed language, it is not a surprise that adding numbers to strings gives a compile-time error. However, one could expect GHC to be unhappy about `"4"`, not about `5` as above! Let's take a deeper look on how the type checker works.
 
-The compiler first encounters the operator `(+)` which has type `Num a => a -> a -> a`. Thus it infers that both arguments of `(+)` should be of the same type `a`. Looking at the first argument `5` does not reveal us what `a` is: it could be `Int`, could be `Double`, etc. But the second argument `4` is certainly `String`, so the compiler concludes that `a` should be `String`.
+The compiler first encounters the operator `(+)` which has type `Num a => a -> a -> a`. Thus it infers that both arguments of `(+)` should be of the same type `a`. Looking at the first argument `5` does not reveal us what `a` is: it could be `Int`, could be `Double`, etc. But the second argument `"4"` is certainly `String`, so the compiler concludes that `a` should be `String`.
 
 Now the compiler looks again at the first argument `5`. Since the second argument of `(+)` is `String`, it infers that `5` must be `String` as well. In Haskell a numeric literal could be anything which has `instance Num`, so GHC searches for `instance Num String` but could not find one and bails out with an error.