[m-rev.] diff: Fix documentation of string.unsafe_prev_index.

[Peter Wang]

library/string.m:
    As above.

diff --git a/library/string.m b/library/string.m
index 8f23b0d64..6c1a5e51c 100644
--- a/library/string.m
+++ b/library/string.m
@@ -291,7 +291,7 @@
     % the code unit `Index'. `CharIndex' is the offset of the beginning of
     % `Char'. Fails if `Index' is zero.
     % WARNING: behavior is UNDEFINED if `Index' is out of range
-    % (negative, or greater than or equal to the length of `String').
+    % (negative, or greater than the length of `String').
     %
 :- pred unsafe_prev_index(string::in, int::in, int::out, char::uo) is semidet.
 
-- 
2.21.0