Merge pull request #118 from f7deleon/fix-bool-value-chapter-6

Fix bool value on chapter 6

src/Tutorial_06_Measure_Bool.lhs

@@ -235,7 +235,7 @@ only invoke the respective functions with non-empty lists.
 Write down a specification for `null` such that `safeHead`
 is verified. Do *not* force `null` to only take non-empty inputs,
 that defeats the purpose. Instead, its type should say that it
-works on *all* lists and returns `True` *if and only if* the input
+works on *all* lists and returns `False` *if and only if* the input
 is non-empty.
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