[string.io] Don't refer to sentry by name it was not given. (#1333)

Author: Eelis

source/strings.tex

@@ -4245,7 +4245,7 @@
 \tcode{is.width(0)}
 is called and the
 \tcode{sentry}
-object \tcode{k} is destroyed.
+object is destroyed.
 
 \pnum
 If the function extracts no characters, it calls
@@ -4328,8 +4328,7 @@
 In any case,
 after the last character is extracted, the
 \tcode{sentry}
-object \tcode{k}
-is destroyed.
+object is destroyed.
 
 \pnum
 If the function extracts no characters, it calls