Fix minor error in README (#1571)

SVG's aren't technically HTML, so I changed:
> Modify the SVGs source HTML
to:
> Modify the SVGs source code

README.md

@@ -80,7 +80,7 @@ Here are some key scripts you'll use during development. Be sure to look to our
 Icons are typically only added by @mdo, but exceptions can be made. New glyphs are designed in Figma first on a 16x16px grid, then exported as flattened SVGs with `fill` (no stroke). Once a new SVG icon has been added to the `icons` directory, we use an npm script to:
 
 1. Optimize our SVGs with SVGO.
-2. Modify the SVGs source HTML, removing all attributes before setting new attributes and values in our preferred order.
+2. Modify the SVGs source code, removing all attributes before setting new attributes and values in our preferred order.
 
 Use `npm run icons` to run the script, run `npm run pages` to build permalink pages, complete those pages, and, finally, commit the results in a new branch for updating.