fix book reference

this is actually part of the Advanced UNIX Programming book by M. Rochkind

Author: vladak

file-api.tex

@@ -883,7 +883,7 @@
 interfaces such as \texttt{utime()} (seconds) or \texttt{utimes()}
 (microseconds).
 \item The shell interface for the above interfaces is the command \texttt{touch}.
-\item There is an excellent section \emph{1.7 Dates and Times} in [Butenhof]
+\item There is an excellent section \emph{1.7 Dates and Times} in [Rochkind]
 that goes through various representations of time in Unix systems, and functions
 that use those.  It is 12 pages of good stuff, highly recommended if you ever
 need to work with time in C and/or Unix.