Fix function name in book: thread_pool_destroy

Page 12 refers to `thread_pool_destroy`, but the actual function
name used in the example code is `tpool_destroy`. This commit
corrects the discrepancy to avoid confusion.

Author: weiso131

concurrency-primer.tex

@@ -529,7 +529,7 @@ \subsection{example}
 \end{ccode}
 
 \textbf{Exchange}
-In function \monobox{thread\_pool\_destroy}, \monobox{atomic\_exchange(\&thrd\_pool->state, cancelled)} reads the current state and replaces it with ``cancelled''. 
+In function \monobox{tpool\_destroy}, \monobox{atomic\_exchange(\&thrd\_pool->state, cancelled)} reads the current state and replaces it with ``cancelled''. 
 A warning message is printed if the pool is destroyed while workers are still ``running''. 
 If the exchange is not performed atomically, we may initially get the state as ``running''. Subsequently, a thread could set the state to ``cancelled'' after finishing the last one, resulting in a false warning.
 
@@ -541,7 +541,7 @@ \subsection{example}
 This indicates that the main thread will wail until the worker completes the job and returns ownership back to the main thread, which ensures correct cooperation.
 
 \textbf{Fetch and…}
-In the function \monobox{thread\_pool\_destroy}, \monobox{atomic\_fetch\_and} is utilized as a means to set the state to ``idle''. 
+In the function \monobox{tpool\_destroy}, \monobox{atomic\_fetch\_and} is utilized as a means to set the state to ``idle''. 
 Yet, in this case, it is not necessary, as the pool needs to be reinitialized for further use regardless.
 Its return value could be further utilized, for instance, to report the previous state and perform additional actions.