webp-container-spec,cosmetics: normalize range syntax

this makes the syntax in this document consistent with
webp-lossless-bitstream-spec.txt

[N-M] -> [N..M]

Bug: webp:448
Change-Id: Iebf39eefb7d3c081a3d10e2804ee215c3aed6b79
diff --git a/doc/webp-container-spec.txt b/doc/webp-container-spec.txt
index 38a3e2c..1b3dabd 100644
--- a/doc/webp-container-spec.txt
+++ b/doc/webp-container-spec.txt
@@ -589,8 +589,8 @@
   * v    otherwise
 
 The final value is derived by adding the decompressed value `X` to the
-predictor and using modulo-256 arithmetic to wrap the \[256-511\] range
-into the \[0-255\] one:
+predictor and using modulo-256 arithmetic to wrap the \[256..511\] range
+into the \[0..255\] one:
 
 `alpha = (predictor + X) % 256`