Fix regexp in customization instructions on readme

Author: tiagofumo

README.md

@@ -70,7 +70,7 @@ let g:NERDTreeExactMatchHighlightColor = {} " this line is needed to avoid error
 let g:NERDTreeExactMatchHighlightColor['.gitignore'] = s:git_orange " sets the color for .gitignore files
 
 let g:NERDTreePatternMatchHighlightColor = {} " this line is needed to avoid error
-let g:NERDTreePatternMatchHighlightColor['.*_spec.rb'] = s:rspec_red " sets the color files ending with _spec.rb
+let g:NERDTreePatternMatchHighlightColor['.*_spec\.rb$'] = s:rspec_red " sets the color files ending with _spec.rb
 
 ```