bumped version number to v1.3.8

doc/zstd_manual.html

@@ -1,10 +1,10 @@
 <html>
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
-<title>zstd 1.3.7 Manual</title>
+<title>zstd 1.3.8 Manual</title>
 </head>
 <body>
-<h1>zstd 1.3.7 Manual</h1>
+<h1>zstd 1.3.8 Manual</h1>
 <hr>
 <a name="Contents"></a><h2>Contents</h2>
 <ol>