fix semigroup mixed answers (#45)

src/quiz-answers/semigroup-first.md

@@ -5,9 +5,9 @@ Is the following semigroup instance lawful (does it respect semigroup laws)?
 ```ts
 import { Semigroup } from 'fp-ts/Semigroup'
 
-/** Always return the second argument */
-const last = <A>(): Semigroup<A> => ({
-  concat: (_first, second) => second
+/** Always return the first argument */
+const first = <A>(): Semigroup<A> => ({
+  concat: (first, _second) => first
 })
 ```
 
@@ -17,5 +17,5 @@ Yes:
 
 - `first`, `second` and the result of `concat` (which is `first`) are all of the same type `A`
 - `concat` is associative:
-  - `concat(concat(first, second), third)` evaluates to `concat(second, third)` which then evaluates to `third`
-  - `concat(first, concat(second, third))` evaluates to `concat(first, third)` which then evaluates to `third`
+  - `concat(concat(first, second), third)` evaluates to `concat(first, third)` which then evaluates to `first`
+  - `concat(first, concat(second, third))` evaluates to `concat(first, second)` which then evaluates to `first`

src/quiz-answers/semigroup-second.md

@@ -15,7 +15,7 @@ const last = <A>(): Semigroup<A> => ({
 
 Yes:
 
-- `first`, `second` and the result of `concat` (which is `first`) are all of the same type `A`
+- `first`, `second` and the result of `concat` (which is `second`) are all of the same type `A`
 - `concat` is associative:
-  - `concat(concat(first, second), third)` evaluates to `concat(first, third)` which then evaluates to `first`
-  - `concat(first, concat(second, third))` evaluates to `concat(first, second)` which then evaluates to `first`
+  - `concat(concat(first, second), third)` evaluates to `concat(second, third)` which then evaluates to `third`
+  - `concat(first, concat(second, third))` evaluates to `concat(first, third)` which then evaluates to `third`