fix broken links (#588)

Author: KappaDistributive

src/concurrency.md

@@ -18,7 +18,7 @@
 [ex-crossbeam-spawn]: concurrency/threads.html#spawn-a-short-lived-thread
 [ex-crossbeam-spsc]: concurrency/threads.html#pass-data-between-two-threads
 [ex-global-mut-state]: concurrency/threads.html#maintain-global-mutable-state
-[ex-threadpool-walk]: concurrency/threads.html#calculate-sha1-sum-of-iso-files-concurrently
+[ex-threadpool-walk]: concurrency/threads.html#calculate-sha256-sum-of-iso-files-concurrently
 [ex-threadpool-fractal]: concurrency/threads.html#draw-fractal-dispatching-work-to-a-thread-pool
 [ex-rayon-iter-mut]: concurrency/parallel.html#mutate-the-elements-of-an-array-in-parallel
 [ex-rayon-any-all]: concurrency/parallel.html#test-in-parallel-if-any-or-all-elements-of-a-collection-match-a-given-predicate

src/science/mathematics.md

@@ -2,7 +2,6 @@
 
 | Recipe | Crates | Categories |
 |--------|--------|------------|
-| [Vector Sum][vector-sum] | [![ndarray-badge]][ndarray] | [![cat-science-badge]][cat-science] |
 | [Vector Norm][vector-norm] | [![ndarray-badge]][ndarray] | [![cat-science-badge]][cat-science] |
 | [Adding matrices][add-matrices] | [![ndarray-badge]][ndarray] | [![cat-science-badge]][cat-science] |
 | [Multiplying matrices][multiply-matrices] | [![ndarray-badge]][ndarray] | [![cat-science-badge]][cat-science] |
@@ -18,7 +17,6 @@
 | [Computing standard deviation][ex-standard-deviation] | [![std-badge]][std] | [![cat-science-badge]][cat-science] |
 | [Big integers][big-integers] | [![num-badge]][num] | [![cat-science-badge]][cat-science] |
 
-[vector-sum]: science/mathematics/linear_algebra.html#vector-sum
 [vector-norm]: science/mathematics/linear_algebra.html#vector-norm
 [add-matrices]: science/mathematics/linear_algebra.html#adding-matrices
 [multiply-matrices]: science/mathematics/linear_algebra.html#multiplying-matrices