Fix _variables.scss filename in README (alshedivat#1683)

Author: raffaem

README.md

@@ -613,7 +613,7 @@ A variety of beautiful theme colors have been selected for you to choose from.
 The default is purple, but you can quickly change it by editing the
 `--global-theme-color` variable in the `_sass/_themes.scss` file.
 Other color variables are listed there as well.
-The stock theme color options available can be found at `_sass/variables.scss`.
+The stock theme color options available can be found at `_sass/_variables.scss`.
 You can also add your own colors to this file assigning each a name for ease of
 use across the template.