Fix readme about config setting

According to current man pages of git-config there is no = to set
the config value.

README.md

@@ -20,7 +20,7 @@ For more information on gitignore: [gitignore(5)][g5]
 git has a global configuration that applies rules to all of
 your projects. For example:
 
-    git config --global core.excludesfile=~/.global_ignore
+    git config --global core.excludesfile ~/.global_ignore
 
 ... will apply the rules in ~/.global_ignore for all of your repos.