Fixed typo in README.md.

Just a 1-character typo fix.

Author: george2

README.md

@@ -56,7 +56,7 @@ The following HTML elements, organized by category, are whitelisted:
 The following attributes, organized by element, are whitelisted:
 
 * a: href (http://, https://, mailto://, github-windows:// and github-mac:// URI schemes and relative paths only)
-* img: src (http:// and https::// URI schemes and relative paths only)
+* img: src (http:// and https:// URI schemes and relative paths only)
 * div: itemscope, itemtype
 * all: abbr, accept, accept-charset, accesskey, action, align, alt, axis, border, cellpadding, cellspacing, char, charoff, charset, checked, cite, clear, cols, colspan, color, compact, coords, datetime, dir, disabled, enctype, for, frame, headers, height, hreflang, hspace, ismap, label, lang, longdesc, maxlength, media, method, multiple, name, nohref, noshade, nowrap, prompt, readonly, rel, rev, rows, rowspan, rules, scope, selected, shape, size, span, start, summary, tabindex, target, title, type, usemap, valign, value, vspace, width, itemprop