fix typo

Author: JunCEEE

EXT_MD.md

@@ -3,7 +3,7 @@ Domain-Specific Naming Conventions for Molecular Dynamics Simulation Codes
 
 openPMD extension name: `MD`
 
-VERSION: 0.0.2 (September 19th, 2019)
+VERSION: 0.0.3 (January 26th, 2020)
 
 Introduction
 ------------
@@ -60,7 +60,7 @@ The naming conventions conforming the naming conventions for [Scalar, Vector and
 
 ### Attributes for each `observable` record
 
-The attributes of unit system for records should be included as defined in [openPMD base standard](https://github.com/openPMD/openPMD-standard/blob/latest/STANDARD.md#unit-systems-and-dimensionality)  .
+The attributes of unit system for records should be included as defined in [openPMD base standard](https://github.com/openPMD/openPMD-standard/blob/latest/STANDARD.md#unit-systems-and-dimensionality).
 
 
 Particle Records