[svn-r22077] Add example of multiple files for h5dump help text.

Tested: local linux

tools/testfiles/tpbitsMaxExceeded.ddl

@@ -1,4 +1,4 @@
-usage: h5dump [OPTIONS] file
+usage: h5dump [OPTIONS] files
   OPTIONS
      -h, --help           Print a usage message and exit
      -n, --contents       Print a list of the file contents and exit
@@ -94,4 +94,8 @@ usage: h5dump [OPTIONS] file
 
       h5dump -d /dset -M 0,1,4,3 quux.h5
 
+  5) Dataset foo in files multi1.h5 multi2.h5 multi3.h5
+
+      h5dump -d /foo multi1.h5 multi2.h5 multi3.h5
+
 h5dump error: Too many masks requested (max. 8). Mask list(0,1,0,1,1,1,2,1,3,1,4,1,5,1,6,1,7,1)