fix invalid URL

Author: wkliao

index.html

@@ -127,13 +127,20 @@ <h2>A Brief History of PnetCDF</h2>
 <h2>A Note About Large File Support</h2>
 <p>
 The classic
-<a href="http://www.unidata.ucar.edu/software/netcdf/old_docs/really_old/guide_15.html">CDF</a>
+<!--
+<a href="http://www.unidata.ucar.edu/software/netcdf/old_docs/really_old/guide_15.html">
+-->
+<a href="doc/c-reference/pnetcdf-c/CDF_002d1-file-format-specification.html">
+CDF</a>
 file format (referred as CDF-1) has been solely in use by NetCDF library
 through version 3.5.1. The classic format has been updated by NASA ESDS
 community standard and added a support for 64-bit offset file format
 (also referred as CDF-2). See
-<a href="https://earthdata.nasa.gov/standards/netcdf-classic">NetCDF Classic and
-64-bit Offset File Formats</a>.
+<!--
+<a href="https://earthdata.nasa.gov/standards/netcdf-classic">
+-->
+<a href="doc/c-reference/pnetcdf-c/CDF_002d2-file-format-specification.html">
+NetCDF Classic and 64-bit Offset File Formats</a>.
 </p>
 <p>
 Starting from 3.6.0, the serial NetCDF library added support for the
@@ -144,7 +151,7 @@ <h2>A Note About Large File Support</h2>
 based largely on work from Greg Sjaardema.</p>
 <p>
 Starting from the release of 0.9.2, PnetCDF supports CDF-2 format. See
-<a href="https://github.com/Parallel-NetCDF/PnetCDF/blob/master/doc/README.large_files">README.large_files</a>
+<a href="https://github.com/Parallel-NetCDF/PnetCDF/blob/master/doc/README.large_files.md">README.large_files</a>
 for more information.
 </p>
 <p>