doc/revisions: remove brackets from rev^-n shorthand

Given that other instances of "{...}" in the revision documentation
represent literal characters of revision specifications, describing
the rev^-n shorthand as "<rev>^-{<n>}" incorrectly suggests that
something like "master^-{1}" is an acceptable form.

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/revisions.txt

@@ -291,7 +291,7 @@ The 'r1{caret}@' notation means all parents of 'r1'.
 The 'r1{caret}!' notation includes commit 'r1' but excludes all of its parents.
 By itself, this notation denotes the single commit 'r1'.
 
-The '<rev>{caret}-{<n>}' notation includes '<rev>' but excludes the <n>th
+The '<rev>{caret}-<n>' notation includes '<rev>' but excludes the <n>th
 parent (i.e. a shorthand for '<rev>{caret}<n>..<rev>'), with '<n>' = 1 if
 not given. This is typically useful for merge commits where you
 can just pass '<commit>{caret}-' to get all the commits in the branch
@@ -333,7 +333,7 @@ Revision Range Summary
   as giving commit '<rev>' and then all its parents prefixed with
   '{caret}' to exclude them (and their ancestors).
 
-'<rev>{caret}-{<n>}', e.g. 'HEAD{caret}-, HEAD{caret}-2'::
+'<rev>{caret}-<n>', e.g. 'HEAD{caret}-, HEAD{caret}-2'::
 	Equivalent to '<rev>{caret}<n>..<rev>', with '<n>' = 1 if not
 	given.