Fix typo in README.md (#91)

The plugin line needed to contain "history-substring-search" (not zsh-history-substring-search) to work for me.

Author: beheist

README.md

@@ -33,7 +33,7 @@ Using [Oh-my-zsh](https://github.com/robbyrussell/oh-my-zsh):
 
 2. Activate the plugin in `~/.zshrc`:
 
-        plugins=( [plugins...] zsh-history-substring-search)
+        plugins=( [plugins...] history-substring-search)
 
 3. Source `~/.zshrc`  to take changes into account: