Update downloads/hdf4/hdf4_3_1.md

Add missing 'n'.

Author: lrknox

downloads/hdf4/hdf4_3_1.md

@@ -29,5 +29,5 @@ Applications that were created with earlier HDF4 releases AND employed internal
 
 Please refer to [Build Instructions](https://github.com/HDFGroup/hdf4/blob/hdf4.3.1/release_notes/INSTALL) for building with either CMake or Autotools.
 
-#### Know Problems:
+#### Known Problems:
 Some SZIP tests in mdfhdf failed when the SZIP encoder is not present and the failure can be ignored at this time.