Fix punctuation.

railties/guides/source/api_documentation_guidelines.textile

@@ -37,7 +37,7 @@ h3. Example Code
 
 Choose meaningful examples that depict and cover the basics as well as interesting points or gotchas.
 
-Use two spaces to indent chunks of code.—that is two spaces with respect to the left margin; the examples
+Use two spaces to indent chunks of code--that is two spaces with respect to the left margin; the examples
 themselves should use "Rails coding conventions":contributing_to_ruby_on_rails.html#follow-the-coding-conventions.
 
 Short docs do not need an explicit "Examples" label to introduce snippets, they just follow paragraphs: