Doc test: use propery regex for file size

The _cat/shards docs asserted that one of the columns looked like
a propery byte size but used a regex like `\d+\.\d+.*` which doesn't
match `0b` which is a possible value. Instead this uses
`\d(\.\d+)?[kmg]?b`.

docs/reference/cat/shards.asciidoc

@@ -22,8 +22,7 @@ twitter 0 p STARTED 3014 31.1mb 192.168.56.10 H5dfFeA
 twitter 0 r UNASSIGNED
 ---------------------------------------------------------------------------
 // TESTRESPONSE[s/3014/\\d+/]
-// TESTRESPONSE[s/31.1/\\d+(\.\\d+)?/]
-// TESTRESPONSE[s/mb/.*/]
+// TESTRESPONSE[s/31.1mb/\\d+(\.\\d+)?[kmg]?b/]
 // TESTRESPONSE[s/192.168.56.10/.*/]
 // TESTRESPONSE[s/H5dfFeA/.*/ _cat]
 
@@ -50,8 +49,7 @@ twitter 0 p STARTED 3014 31.1mb 192.168.56.10 H5dfFeA
 twitter 0 r UNASSIGNED
 ---------------------------------------------------------------------------
 // TESTRESPONSE[s/3014/\\d+/]
-// TESTRESPONSE[s/31.1/\\d+\.\\d+/]
-// TESTRESPONSE[s/mb/.*/]
+// TESTRESPONSE[s/31.1mb/\\d+(\.\\d+)?[kmg]?b/]
 // TESTRESPONSE[s/192.168.56.10/.*/]
 // TESTRESPONSE[s/H5dfFeA/.*/ _cat]