Correct broken helptags command in docs.

[curran]

Currently, the documentation states:

Then reload vim, run :helptags, and check out :help NERD_tree.txt.

It turns out that :helptags is not a valid Vim command. I got the error

E471: Argument required

This experience is also described in this StackOverflow post - Vim - helptags not working for NERDtree.

I propose to change the README to say

Then reload vim, run :helptags ~/.vim/bundle/nerdtree/doc/, and check out :help NERD_tree.txt.

This is the valid command that needs to be executed. This will help people install NERDTree, and avoid the installation frustration that I and many others experienced.

[curran]

@PhilRunninger Thanks for merging!