wording nit

toc.ftr

 
 
 
-The source listing is preceded by a cross-reference listing every defined 
+The source listing is preceded by a cross-reference that lists every defined 
 constant, struct, global variable, and function in xv6.  Each entry gives,
 on the same line as the name, the line number (or, in a few cases, numbers)
 where the name is defined.  Successive lines in an entry list the line