Fix spelling of "occurrence" where appropriate.

Author: lanurmi

doc/rationale

@@ -4,12 +4,12 @@ implement generic composition classes. The former was choosen,
 mostly because of simplicity.
 
 There were two implementation approaches for multiple option
-occurences in options_and_arguments. First is store them
+occurrences in options_and_arguments. First is store them
 separately. The advantage is that it's easy to obtain all
-occurences before certain position on command line. The
+occurrences before certain position on command line. The
 disadvantage is that we cannot return a reference to 
 vector<vector<string> > in get_all_values. It was considered
 that if support for position-dependent options is to be
-added, then we're be mostly interested in occurences of 
+added, then we're be mostly interested in occurrences of
 a single option that were before some point. That's possible
-with vector<vector<string> > storage.
+with vector<vector<string> > storage.

doc/rationale.dox

@@ -81,7 +81,7 @@
 
     @section help_handling Handling of --help
 
-    It was suggested by Gennadiy Rozental that occurence of <tt>--help</tt>
+    It was suggested by Gennadiy Rozental that occurrence of <tt>--help</tt>
     on command line results in throwing an exception. Actually, the 
     &quot;special&quot; option must have been configurable. This was not 
     implemented, because applications might reasonable want to process

src/options_description.cpp

@@ -8,7 +8,7 @@
 #define BOOST_PROGRAM_OPTIONS_SOURCE
 #include <boost/program_options/config.hpp>
 #include <boost/program_options/options_description.hpp>
-// FIXME: this is only to get multiple_occurences class
+// FIXME: this is only to get multiple_occurrences class
 // should move that to a separate headers.
 #include <boost/program_options/parsers.hpp>