Fix typos in doc

Fixes upstream PR #148, #184

doc/source/documentation/excellon.rst

@@ -11,7 +11,7 @@ information. The Excellon format is used to program CNC drilling macines for
 drilling holes in PCBs. As such, excellon files are sometimes refererred to as
 NC-drill files.  The Excellon format reference is available 
 `here <http://www.excellon.com/manuals/program.htm>`_. The :mod:`excellon`
-submodule implements calsses to read and write excellon files without having
+submodule implements classes to read and write excellon files without having
 to know the precise details of the format.
 
 The :mod:`excellon` submodule's :func:`read` function serves as a

doc/source/documentation/rs274x.rst

@@ -9,7 +9,7 @@
 The RS-274X (Gerber) format is the most common format for exporting PCB
 artwork.  The Specification is published by Ucamco and is available
 `here <http://www.ucamco.com/files/downloads/file/81/the_gerber_file_format_specification.pdf>`_.
-The :mod:`rs274x` submodule implements calsses to read and write
+The :mod:`rs274x` submodule implements classes to read and write
 RS-274X files without having to know the precise details of the format.
 
 The :mod:`rs274x` submodule's :func:`read` function serves as a