Update lambda-calculus.md

Fix #5

doc/lambda-calculus.md

@@ -231,7 +231,7 @@ As expected, the result is `1`.
 λ is a small but expressive language. It has only three types of expressions: variables, abstractions, and function applications. Even without built-ins, we can express both simple and complex data types including integers, booleans, and records. Evaluation of λ is equally simple, with only three reduction rules: Β-reduction, α-conversion, and η-conversion.
 
 # References
-1. [Lambda Calculus](https://en.wikipedia.org/wiki/Lambd.a_calculus). Wikipedia: The Free Encyclopedia
+1. [Lambda Calculus](https://en.wikipedia.org/wiki/Lambda_calculus). Wikipedia: The Free Encyclopedia
 2. Types and Programming Languages. Benjamin C. Pierce
 3. [Simply typed lambda calculus](https://en.wikipedia.org/wiki/Simply_typed_lambda_calculus). Wikipedia: The Free Encyclopedia
 4. [System F](https://en.wikipedia.org/wiki/System_F). Wikipedia: The Free Encyclopedia