Documentation: do not assume that n > 1 in <rev>~$n

We explained <rev>~<n> as <n>th generation grand-parent, but a reader got
confused by the "grand-" part when <n> is 1.

Reword it with "ancestor"; with the "generation" and "following only the
first parents" around there, what we try to describe should be clear
enough now.

Noticed-by: Luke Diamand <luke@diamand.org>
Helped-by: Thomas Rast <trast@inf.ethz.ch>
Helped-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -101,7 +101,7 @@ the '$GIT_DIR/refs' directory or from the '$GIT_DIR/packed-refs' file.
 
 '<rev>{tilde}<n>', e.g. 'master{tilde}3'::
   A suffix '{tilde}<n>' to a revision parameter means the commit
-  object that is the <n>th generation grand-parent of the named
+  object that is the <n>th generation ancestor of the named
   commit object, following only the first parents.  I.e. '<rev>{tilde}3' is
   equivalent to '<rev>{caret}{caret}{caret}' which is equivalent to
   '<rev>{caret}1{caret}1{caret}1'.  See below for an illustration of