Fix typo in docs (#1306)

README.markdown

@@ -90,7 +90,7 @@ vim -u NONE -c "helptags ~/.vim/pack/vendor/start/nerdtree/doc" -c q
 After installing NERDTree, the best way to learn it is to turn on the Quick Help. Open NERDTree with the `:NERDTree` command, and press `?` to turn on the Quick Help, which will show you all the mappings and commands available in the NERDTree. Of course, your most complete source of information is the documentation: `:help NERDTree`.
 
 ## NERDTree Plugins
-NERDTree can be extended with custom mappings and functions using its built-in API. The details of this API and are described in the included documentation. Several plugins have been written, and are available on Github for installation like any other plugin. The plugins in this list are maintained (or not) by their respective owners, and certain combinations may be incompatible.
+NERDTree can be extended with custom mappings and functions using its built-in API. The details of this API are described in the included documentation. Several plugins have been written, and are available on Github for installation like any other plugin. The plugins in this list are maintained (or not) by their respective owners, and certain combinations may be incompatible.
 
 * [Xuyuanp/nerdtree-git-plugin](https://github.com/Xuyuanp/nerdtree-git-plugin): Shows Git status flags for files and folders in NERDTree.
 * [ryanoasis/vim-devicons](https://github.com/ryanoasis/vim-devicons): Adds filetype-specific icons to NERDTree files and folders,