docs: Update filenames in keep_data_small.txt

The filenames in docs/keep_data_small.txt are a little bit outdated.
It's better to change it to the current name.

decompress_unzip.c -> decompress_gunzip.c
(since commit 774bce8e8ba1e424c953e8f13aee8f0778c8a911)
libbb/messages.c -> libbb/ptr_to_globals.c
(since commit 574f2f43948bb21d6e4187936ba5a5afccba25f6)

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

docs/keep_data_small.txt

@@ -59,7 +59,7 @@ wait
 		Example 1
 
 One example how to reduce global data usage is in
-archival/libarchive/decompress_unzip.c:
+archival/libarchive/decompress_gunzip.c:
 
 /* This is somewhat complex-looking arrangement, but it allows
  * to place decompressor state either in bss or in
@@ -87,7 +87,7 @@ take a look at archival/gzip.c. Here all global data is replaced by
 single global pointer (ptr_to_globals) to allocated storage.
 
 In order to not duplicate ptr_to_globals in every applet, you can
-reuse single common one. It is defined in libbb/messages.c
+reuse single common one. It is defined in libbb/ptr_to_globals.c
 as struct globals *const ptr_to_globals, but the struct globals is
 NOT defined in libbb.h. You first define your own struct: